add parameter --time

You can not generate a code for a specific time. Currently only UTC is
supported.

build.zig.zon

@@ -1,13 +1,27 @@
 .{
-    .name = "zig-totp",
+    .name = .f,
     // This is a [Semantic Version](https://semver.org/).
     // In a future version of Zig it will be used for package deduplication.
     .version = "0.1.0-dev",
 
+    // Together with name, this represents a globally unique package
+    // identifier. This field is generated by the Zig toolchain when the
+    // package is first created, and then *never changes*. This allows
+    // unambiguous detection of one package being an updated version of
+    // another.
+    //
+    // When forking a Zig project, this id should be regenerated (delete the
+    // field and run `zig build`) if the upstream project is still maintained.
+    // Otherwise, the fork is *hostile*, attempting to take control over the
+    // original project's identity. Thus it is recommended to leave the comment
+    // on the following line intact, so that it shows up in code reviews that
+    // modify the field.
+    .fingerprint = 0x76d32be0e41821b4, // Changing this has security and trust implications.
+
     // This field is optional.
     // This is currently advisory only; Zig does not yet do anything
     // with this value.
-    //.minimum_zig_version = "0.11.0",
+    //.minimum_zig_version = "0.14.0",
 
     // This field is optional.
     // Each dependency must either provide a `url` and `hash`, or a `path`.

src/main.zig

@@ -11,6 +11,8 @@ const ArrayList = std.ArrayList;
 const Base32 = @import("base32.zig").Base32;
 const Base32Error = @import("base32.zig").Base32Error;
 const options = @import("config");
+const Time = @import("time.zig").Time;
+const TimeError = @import("time.zig").TimeError;
 
 // otpauth://totp/AWS+Dev?secret=47STA47VFCMMLLWOLHWO3KY7MYNC36MLCDTHOLIYKJCTTSSAMKVM7YA3VWT2AJEP&digits=6&icon=Amazon
 // otpauth://totp/Gitea%20%28git.lucares.de%29:andi?algorithm=SHA1&digits=6&issuer=Gitea&period=30&secret=MSP53Q672UJMSCLQVRCJKMMZKK7MWSMYFL77OQ24JBM65RZWY7F2Y45FMTNLYM36
@@ -33,6 +35,9 @@ pub fn main() !void {
             ArgumentError.AuthenticatorNotFound => {
                 try std.io.getStdErr().writer().print("authenticator not found\n", .{});
             },
+            TimeError.ParsingError => {
+                try std.io.getStdErr().writer().print("Invalid time. The argument for the --time parameter must be a valid ISO 8601 date with second precision for timezone UTC. E.g. 2000-01-01T00:00:00\n", .{});
+            },
             else => |leftover_err| {
                 try std.io.getStdErr().writer().print("{?}\n", .{leftover_err});
             },
@@ -60,11 +65,13 @@ fn mainInternal() !void {
         }
     } else if (arg.show != null) {
         const authenticator = try getAuthenticator(allocator, arg.show.?, arg.config_location);
-        const code = try authenticator.code(allocator, std.time.timestamp);
+        const time = if (arg.time != null) try Time.parseIso8601(arg.time.?) else std.time.timestamp();
+        const code = try authenticator.code(allocator, time);
         try std.io.getStdOut().writer().print("{s}\n", .{code});
     } else if (arg.validateAuthenticator != null and arg.validateCode != null) {
         const authenticator = try getAuthenticator(allocator, arg.validateAuthenticator.?, arg.config_location);
-        const matches = try authenticator.validate(allocator, arg.validateCode.?, std.time.timestamp);
+        const time = if (arg.time != null) try Time.parseIso8601(arg.time.?) else std.time.timestamp();
+        const matches = try authenticator.validate(allocator, arg.validateCode.?, time);
         if (!matches) {
             std.process.exit(1);
         }
@@ -91,13 +98,14 @@ fn printHelp() void {
         \\  --fish                 Generate shell integration for fish. Usage:
         \\                              zig-totp --fish > $__fish_config_dir/completions/zig-totp.fish
         \\  --help                 Show this help 
+        \\  --time                 Set the time at which a code is computed. Time format is ISO 8601 with second precision for timezone UTC, e.g., 2000-01-01T00:00:00
         \\  --version              Print the version number 
         \\
     ;
     std.debug.print("{s}", .{msg});
 }
 
-const Args = struct { list: bool, show: ?[]const u8, config_location: []const u8, validateAuthenticator: ?[]const u8, validateCode: ?[]const u8, print_version: bool, show_help: bool };
+const Args = struct { list: bool, show: ?[]const u8, config_location: []const u8, validateAuthenticator: ?[]const u8, validateCode: ?[]const u8, print_version: bool, show_help: bool, time: ?[]const u8 };
 
 const OtpAuthUrl = struct { name: []const u8, secretEncoded: []const u8, url: []const u8, period: u32, digits: u4, algorithm: []const u8 };
 
@@ -105,7 +113,7 @@ const Authenticator = struct {
     url: OtpAuthUrl,
 
     /// algorithm based on https://www.rfc-editor.org/rfc/rfc6238
-    pub fn code(self: Authenticator, allocator: Allocator, timeFunc: *const fn () i64) ![]const u8 {
+    pub fn code(self: Authenticator, allocator: Allocator, time: i64) ![]const u8 {
         if (self.url.name.len > 0) {}
 
         const secret = try Base32.decodeU8(allocator, self.url.secretEncoded);
@@ -114,7 +122,7 @@ const Authenticator = struct {
             debug("secret: {s}\n", .{secret});
         }
 
-        const intervalNumber = @divTrunc(timeFunc(), self.url.period);
+        const intervalNumber = @divTrunc(time, self.url.period);
 
         var intervalAsU8Array: [8]u8 = undefined;
         std.mem.writePackedInt(i64, intervalAsU8Array[0..], 0, intervalNumber, .big);
@@ -160,8 +168,8 @@ const Authenticator = struct {
         return result;
     }
 
-    fn validate(self: Authenticator, allocator: Allocator, expectedCode: []const u8, timeFunc: *const fn () i64) !bool {
+    fn validate(self: Authenticator, allocator: Allocator, expectedCode: []const u8, time: i64) !bool {
-        const actualCode = try self.code(allocator, timeFunc);
+        const actualCode = try self.code(allocator, time);
         defer allocator.free(actualCode);
         return eql(u8, actualCode, expectedCode);
     }
@@ -193,7 +201,7 @@ fn zeroPad(allocator: Allocator, digits: u4, x: anytype) ![]u8 {
 const ArgumentError = error{ InvalidOtpAuthUrl, UnknownParameter, MissingConfigLocation, AuthenticatorNotFound, MissingAuthenticatorParam, FailedToOpenConfigFile, TooManyParsinErrors };
 
 fn parseArgs(allocator: Allocator, args: []const []const u8) !Args {
-    var result = Args{ .list = false, .show = null, .config_location = try configLocation(allocator), .validateAuthenticator = null, .validateCode = null, .print_version = false, .show_help = false };
+    var result = Args{ .list = false, .show = null, .config_location = try configLocation(allocator), .validateAuthenticator = null, .validateCode = null, .print_version = false, .show_help = false, .time = null };
     var i: u17 = 1;
     while (i < args.len) : (i += 1) {
         const arg = args[i];
@@ -212,7 +220,7 @@ fn parseArgs(allocator: Allocator, args: []const []const u8) !Args {
                 \\   then 
                 \\      COMPREPLY=($(compgen -A file -- "${COMP_WORDS[$COMP_CWORD]}" ))
                 \\   else 
-                \\      COMPREPLY=($(compgen -W "list show validate --bash --fish --help --config" -- "${COMP_WORDS[$COMP_CWORD]}" ))
+                \\      COMPREPLY=($(compgen -W "list show validate --bash --fish --help --time --config" -- "${COMP_WORDS[$COMP_CWORD]}" ))
                 \\   fi
                 \\}
                 \\complete -F _zig_totp_completions zig-totp
@@ -231,7 +239,8 @@ fn parseArgs(allocator: Allocator, args: []const []const u8) !Args {
                 \\# parameters:
                 \\complete -c zig-totp -l bash -d "generate bash integration code"
                 \\complete -c zig-totp -l fish -d "generate fish integration code"
-                \\complete -c zig-totp -l config -d "define config location" --force-files --require-parameter -d 'override config location'
+                \\complete -c zig-totp -l config --force-files --require-parameter -d 'override config location'
+                \\complete -c zig-totp -l time --require-parameter -d 'set time'
                 \\complete -c zig-totp -l help -d "show help"
                 \\
                 \\# sub commands:
@@ -250,7 +259,7 @@ fn parseArgs(allocator: Allocator, args: []const []const u8) !Args {
         } else if (eql(u8, "--config", arg)) {
             i += 1;
             if (i >= args.len) {
-                std.debug.print("expected a path after parameter '--config'\n  see 'zig-totp' for help\n", .{});
+                std.debug.print("expected a path after parameter '--config'\n  see 'zig-totp --help' for help\n", .{});
                 return error.MissingConfigLocation;
             }
             result.config_location = args[i];
@@ -258,6 +267,13 @@ fn parseArgs(allocator: Allocator, args: []const []const u8) !Args {
             result.show_help = true;
         } else if (eql(u8, "--version", arg)) {
             result.print_version = true;
+        } else if (eql(u8, "--time", arg)) {
+            i += 1;
+            if (i >= args.len) {
+                std.debug.print("expected a path after parameter '--time'\n  see 'zig-totp --help' for help\n", .{});
+                return error.MissingConfigLocation;
+            }
+            result.time = args[i];
         } else if (eql(u8, "list", arg)) {
             result.list = true;
         } else if (eql(u8, "show", arg)) {
@@ -515,14 +531,14 @@ test "zero padding" {
 test "authenticator generate code with 6 digits" {
     const authenticator = Authenticator{ .url = OtpAuthUrl{ .name = "TestAuthenticator", .secretEncoded = "HJ5PX4IFQKD37HFNXLJYBAVD5G6NMMUOKUCDXJ4XTLUUBWSXRXEN5SR4MLAZA5M2", .url = "not needed", .period = 30, .digits = 6, .algorithm = "SHA1" } };
 
-    const code = try authenticator.code(std.testing.allocator, _closure_return_1725695340);
+    const code = try authenticator.code(std.testing.allocator, 1725695340);
     defer std.testing.allocator.free(code);
     try std.testing.expectEqualStrings("218139", code);
 }
 test "authenticator generate code with 10 digits and zero padding" {
     const authenticator = Authenticator{ .url = OtpAuthUrl{ .name = "TestAuthenticator", .secretEncoded = "HJ5PX4IFQKD37HFNXLJYBAVD5G6NMMUOKUCDXJ4XTLUUBWSXRXEN5SR4MLAZA5M2", .url = "not needed", .period = 60, .digits = 10, .algorithm = "SHA1" } };
 
-    const code = try authenticator.code(std.testing.allocator, _closure_return_1725695340);
+    const code = try authenticator.code(std.testing.allocator, 1725695340);
     defer std.testing.allocator.free(code);
     try std.testing.expectEqualStrings("0844221464", code);
 }
@@ -534,117 +550,89 @@ test "testcases from https://www.rfc-editor.org/rfc/rfc6238#appendix-A" {
 
     const secretForSha512 = "GEZDGNBVGY3TQOJQGEZDGNBVGY3TQOJQGEZDGNBVGY3TQOJQGEZDGNBVGY3TQOJQGEZDGNBVGY3TQOJQGEZDGNBVGY3TQOJQGEZDGNA"; // hex: "313233343536373839303132333435363738393031323334353637383930313233343536373839303132333435363738393031323334353637383930"
 
-    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8, .algorithm = "SHA1" }, _closure_return_59, "94287082");
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8, .algorithm = "SHA1" }, 59, "94287082");
 
-    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8, .algorithm = "SHA1" }, _closure_return_1111111109, "07081804");
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8, .algorithm = "SHA1" }, 1111111109, "07081804");
 
-    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8, .algorithm = "SHA1" }, _closure_return_1111111111, "14050471");
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8, .algorithm = "SHA1" }, 1111111111, "14050471");
 
-    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8, .algorithm = "SHA1" }, _closure_return_1234567890, "89005924");
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8, .algorithm = "SHA1" }, 1234567890, "89005924");
 
-    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8, .algorithm = "SHA1" }, _closure_return_2000000000, "69279037");
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8, .algorithm = "SHA1" }, 2000000000, "69279037");
 
-    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8, .algorithm = "SHA1" }, _closure_return_20000000000, "65353130");
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8, .algorithm = "SHA1" }, 20000000000, "65353130");
 
-    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha256, .url = "", .period = 30, .digits = 8, .algorithm = "SHA256" }, _closure_return_59, "46119246");
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha256, .url = "", .period = 30, .digits = 8, .algorithm = "SHA256" }, 59, "46119246");
 
-    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha256, .url = "", .period = 30, .digits = 8, .algorithm = "SHA256" }, _closure_return_1111111109, "68084774");
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha256, .url = "", .period = 30, .digits = 8, .algorithm = "SHA256" }, 1111111109, "68084774");
 
-    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha256, .url = "", .period = 30, .digits = 8, .algorithm = "SHA256" }, _closure_return_1111111111, "67062674");
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha256, .url = "", .period = 30, .digits = 8, .algorithm = "SHA256" }, 1111111111, "67062674");
 
-    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha256, .url = "", .period = 30, .digits = 8, .algorithm = "SHA256" }, _closure_return_1234567890, "91819424");
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha256, .url = "", .period = 30, .digits = 8, .algorithm = "SHA256" }, 1234567890, "91819424");
 
-    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha256, .url = "", .period = 30, .digits = 8, .algorithm = "SHA256" }, _closure_return_2000000000, "90698825");
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha256, .url = "", .period = 30, .digits = 8, .algorithm = "SHA256" }, 2000000000, "90698825");
 
-    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha256, .url = "", .period = 30, .digits = 8, .algorithm = "SHA256" }, _closure_return_20000000000, "77737706");
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha256, .url = "", .period = 30, .digits = 8, .algorithm = "SHA256" }, 20000000000, "77737706");
 
-    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha512, .url = "", .period = 30, .digits = 8, .algorithm = "SHA512" }, _closure_return_59, "90693936");
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha512, .url = "", .period = 30, .digits = 8, .algorithm = "SHA512" }, 59, "90693936");
 
-    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha512, .url = "", .period = 30, .digits = 8, .algorithm = "SHA512" }, _closure_return_1111111109, "25091201");
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha512, .url = "", .period = 30, .digits = 8, .algorithm = "SHA512" }, 1111111109, "25091201");
 
-    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha512, .url = "", .period = 30, .digits = 8, .algorithm = "SHA512" }, _closure_return_1111111111, "99943326");
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha512, .url = "", .period = 30, .digits = 8, .algorithm = "SHA512" }, 1111111111, "99943326");
 
-    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha512, .url = "", .period = 30, .digits = 8, .algorithm = "SHA512" }, _closure_return_1234567890, "93441116");
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha512, .url = "", .period = 30, .digits = 8, .algorithm = "SHA512" }, 1234567890, "93441116");
 
-    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha512, .url = "", .period = 30, .digits = 8, .algorithm = "SHA512" }, _closure_return_2000000000, "38618901");
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha512, .url = "", .period = 30, .digits = 8, .algorithm = "SHA512" }, 2000000000, "38618901");
 
-    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha512, .url = "", .period = 30, .digits = 8, .algorithm = "SHA512" }, _closure_return_20000000000, "47863826");
+    try testTOTP(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha512, .url = "", .period = 30, .digits = 8, .algorithm = "SHA512" }, 20000000000, "47863826");
 }
 
-fn testTOTP(otpAuthUrl: OtpAuthUrl, timeFunc: *const fn () i64, expected: []const u8) !void {
+fn testTOTP(otpAuthUrl: OtpAuthUrl, time: i64, expected: []const u8) !void {
     const authenticator = Authenticator{ .url = otpAuthUrl };
 
-    const code = try authenticator.code(std.testing.allocator, timeFunc);
+    const code = try authenticator.code(std.testing.allocator, time);
     defer std.testing.allocator.free(code);
     try std.testing.expectEqualStrings(expected, code);
 }
 
-fn _closure_return_1725695340() i64 {
-    return 1725695340;
-}
-
-fn _closure_return_59() i64 {
-    return 59;
-}
-
-fn _closure_return_1111111109() i64 {
-    return 1111111109;
-}
-
-fn _closure_return_1111111111() i64 {
-    return 1111111111;
-}
-
-fn _closure_return_1234567890() i64 {
-    return 1234567890;
-}
-
-fn _closure_return_2000000000() i64 {
-    return 2000000000;
-}
-
-fn _closure_return_20000000000() i64 {
-    return 20000000000;
-}
-
 test "secret is not Base32 - invalid padding" {
     const authenticator = Authenticator{ .url = OtpAuthUrl{ .name = "", .secretEncoded = "H", .url = "", .period = 60, .digits = 6, .algorithm = "SHA1" } };
 
-    const actualError = authenticator.code(std.testing.allocator, _closure_return_1725695340);
+    const actualError = authenticator.code(std.testing.allocator, 1725695340);
     try std.testing.expectError(Base32Error.InvalidPadding, actualError);
 }
 test "secret is not Base32 - invalid padding, input has too many bytes" {
     const authenticator = Authenticator{ .url = OtpAuthUrl{ .name = "", .secretEncoded = "HAX", .url = "", .period = 60, .digits = 6, .algorithm = "SHA1" } };
 
-    const actualError = authenticator.code(std.testing.allocator, _closure_return_1725695340);
+    const actualError = authenticator.code(std.testing.allocator, 1725695340);
     try std.testing.expectError(Base32Error.InvalidPadding, actualError);
 }
 test "secret is not Base32 - invalid padding, trailing bits are not 0" {
     const authenticator = Authenticator{ .url = OtpAuthUrl{ .name = "", .secretEncoded = "HX", .url = "", .period = 60, .digits = 6, .algorithm = "SHA1" } };
 
-    const actualError = authenticator.code(std.testing.allocator, _closure_return_1725695340);
+    const actualError = authenticator.code(std.testing.allocator, 1725695340);
     try std.testing.expectError(Base32Error.InvalidPadding, actualError);
 }
 
 test "secret is not Base32 - invalid character" {
     const authenticator = Authenticator{ .url = OtpAuthUrl{ .name = "", .secretEncoded = "0", .url = "", .period = 60, .digits = 6, .algorithm = "SHA1" } };
 
-    const actualError = authenticator.code(std.testing.allocator, _closure_return_1725695340);
+    const actualError = authenticator.code(std.testing.allocator, 1725695340);
     try std.testing.expectError(Base32Error.InvalidCharacter, actualError);
 }
 
 test "testcases for 'validate'" {
     const secretForSha1 = "GEZDGNBVGY3TQOJQGEZDGNBVGY3TQOJQ"; // plain: "12345678901234567890"  hex: "3132333435363738393031323334353637383930"
 
-    try testValidate(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8, .algorithm = "SHA1" }, _closure_return_59, "94287082", true);
+    try testValidate(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8, .algorithm = "SHA1" }, 59, "94287082", true);
 
-    try testValidate(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8, .algorithm = "SHA1" }, _closure_return_1111111109, "07081804", true);
+    try testValidate(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8, .algorithm = "SHA1" }, 1111111109, "07081804", true);
 
-    try testValidate(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8, .algorithm = "SHA1" }, _closure_return_1111111109, "12345678", false);
+    try testValidate(OtpAuthUrl{ .name = "", .secretEncoded = secretForSha1, .url = "", .period = 30, .digits = 8, .algorithm = "SHA1" }, 1111111109, "12345678", false);
 }
-fn testValidate(otpAuthUrl: OtpAuthUrl, timeFunc: *const fn () i64, expected: []const u8, expectedMatch: bool) !void {
+fn testValidate(otpAuthUrl: OtpAuthUrl, time: i64, expected: []const u8, expectedMatch: bool) !void {
     const authenticator = Authenticator{ .url = otpAuthUrl };
 
-    const match = try authenticator.validate(std.testing.allocator, expected, timeFunc);
+    const match = try authenticator.validate(std.testing.allocator, expected, time);
     try std.testing.expectEqual(expectedMatch, match);
 }

src/time.zig

@@ -0,0 +1,102 @@
+const std = @import("std");
+
+const expect = std.testing.expect;
+const expectEqualStrings = std.testing.expectEqualStrings;
+const expectEqual = std.testing.expectEqual;
+const Allocator = std.mem.Allocator;
+
+pub const TimeError = error{ParsingError};
+
+pub const Time = struct {
+    /// expecting a date in the format yyyy-MM-dd'T'HH:mm:ss
+    /// returning a unix timestamp
+    /// does not handle leap seconds
+    /// only handles UTC
+    pub fn parseIso8601(data: []const u8) !i64 {
+        const format = "nnnn-nn-nnTnn:nn:nn";
+        //              0123456789012345678
+
+        // validate the format
+        if (data.len != 19) {
+            return TimeError.ParsingError;
+        }
+
+        for (format, 0..) |formatChar, index| {
+            if (index >= data.len) {
+                return TimeError.ParsingError;
+            }
+
+            const c = data[index];
+            switch (formatChar) {
+                'n' => {
+                    if (!isDigit(c)) return TimeError.ParsingError;
+                },
+                else => {
+                    if (formatChar != c) return TimeError.ParsingError;
+                },
+            }
+        }
+
+        const year = try std.fmt.parseInt(i64, data[0..4], 10);
+        const month = try std.fmt.parseInt(i64, data[5..7], 10);
+        const day = try std.fmt.parseInt(i64, data[8..10], 10);
+        const hour = try std.fmt.parseInt(i64, data[11..13], 10);
+        const minute = try std.fmt.parseInt(i64, data[14..16], 10);
+        const second = try std.fmt.parseInt(i64, data[17..19], 10);
+
+        const unixTime: i64 = (year - 1970) * 365 * 24 * 3600 //
+        + leapDays(year, month) * 24 * 3600 //
+        + dayInYear(month, day) * 24 * 3600 //
+        + hour * 3600 //
+        + minute * 60 //
+        + second;
+
+        return unixTime;
+    }
+
+    fn isDigit(byte: u8) bool {
+        return byte >= '0' and byte <= '9';
+    }
+
+    fn dayInYear(month: i64, day: i64) i64 {
+        const daysPerMonth = [_]u8{ 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
+        var result: i64 = 0;
+        var i: u8 = 0;
+        while (i < (month - 1)) : (i += 1) {
+            result += daysPerMonth[i];
+        }
+        result += day - 1;
+        return result;
+    }
+    fn leapDays(year: i64, month: i64) i64 {
+        var i: u64 = 1972;
+        var result: i64 = 0;
+        while (i < year) : (i += 1) {
+            if (@mod(i, 4) == 0 and (i % 100 != 0 or @mod(i, 400) == 0)) {
+                result += 1;
+            }
+        }
+        if (month > 2 and @mod(year, 4) == 0 and (@mod(year, 100) != 0 or @mod(year, 400) == 0)) {
+            result += 1;
+        }
+        return result;
+    }
+};
+
+test "parsing error 'x025-01-01T00:00:00'" {
+    try std.testing.expectError(TimeError.ParsingError, Time.parseIso8601("x025-01-01T00:00:00"));
+}
+
+test "parsing error '2025-01-01'" {
+    try std.testing.expectError(TimeError.ParsingError, Time.parseIso8601("2025-01-01T"));
+}
+
+//test "parsing validation successfull" {
+//    const y = try Time.parseIso8601("2025-12-13T01:02:03");
+//    try expectEqual(12, y);
+//}
+
+test "parse 2025-02-18T18:37:45, expecting 1739903865" {
+    const unixTime = try Time.parseIso8601("2025-02-18T18:37:45");
+    try expectEqual(1739903865, unixTime);
+}