replace the dateTo with an interval

I hope an interval is easier to handle than having 
to change two date fields

pdb-plotting/src/main/java/org/lucares/pdb/plot/api/PlotSettings.java

@@ -5,6 +5,7 @@ import java.time.LocalDateTime;
 import java.time.OffsetDateTime;
 import java.time.ZoneOffset;
 import java.time.format.DateTimeFormatter;
+import java.time.temporal.ChronoUnit;
 
 import org.apache.commons.lang3.StringUtils;
 
@@ -26,7 +27,7 @@ public class PlotSettings {
 
 	private String dateFrom;
 
-	private String dateTo;
+	private String dateRange;
 
 	public String getQuery() {
 		return query;
@@ -84,12 +85,12 @@ public class PlotSettings {
 		this.dateFrom = dateFrom;
 	}
 
-	public String getDateTo() {
-		return dateTo;
+	public String getDateRange() {
+		return dateRange;
 	}
 
-	public void setDateTo(final String dateTo) {
-		this.dateTo = dateTo;
+	public void setDateRange(final String dateRange) {
+		this.dateRange = dateRange;
 	}
 
 	public OffsetDateTime dateFrom() {
@@ -103,10 +104,40 @@ public class PlotSettings {
 	}
 
 	public OffsetDateTime dateTo() {
-		if (StringUtils.isEmpty(dateTo)) {
+
+		final int period = Integer.parseInt(dateRange.split(" ")[0]);
+		final ChronoUnit unit = toChronoUnit(dateRange.split(" ")[1]);
+
+		if (StringUtils.isEmpty(dateRange)) {
 			return OffsetDateTime.ofInstant(Instant.ofEpochMilli(Long.MAX_VALUE), ZoneOffset.UTC);
 		} else {
-			return LocalDateTime.parse(dateTo, DATE_FORMAT).atOffset(ZoneOffset.UTC);
+			return dateFrom().plus(period, unit);
+		}
+	}
+
+	private ChronoUnit toChronoUnit(final String string) {
+
+		switch (string) {
+		case "second":
+		case "seconds":
+			return ChronoUnit.SECONDS;
+		case "minute":
+		case "minutes":
+			return ChronoUnit.MINUTES;
+		case "hour":
+		case "hours":
+			return ChronoUnit.HOURS;
+		case "day":
+		case "days":
+			return ChronoUnit.DAYS;
+		case "week":
+		case "weeks":
+			return ChronoUnit.WEEKS;
+		case "month":
+		case "months":
+			return ChronoUnit.MONTHS;
+		default:
+			throw new IllegalArgumentException(string + " is an unknown chrono unit");
 		}
 	}
 }