replace the dateTo with an interval

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

pdb-ui/src/main/java/org/lucares/pdbui/PlotSettingsTransformer.java

@@ -17,7 +17,7 @@ class PlotSettingsTransformer {
 		result.setLimit(request.getLimit());
 		result.setLimitBy(toLimit(request.getLimitBy()));
 		result.setDateFrom(request.getDateFrom());
-		result.setDateTo(request.getDateTo());
+		result.setDateRange(request.getDateRange());
 
 		return result;
 	}

pdb-ui/src/main/java/org/lucares/pdbui/domain/PlotRequest.java

@@ -15,7 +15,7 @@ public class PlotRequest {
 
 	private String dateFrom;
 
-	private String dateTo;
+	private String dateRange;
 
 	public String getQuery() {
 		return query;
@@ -78,11 +78,15 @@ public class PlotRequest {
 		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) {
+		if (!dateRange.matches("\\d+ (second|minute|hour|day|week|month)s?")) {
+			throw new IllegalArgumentException(dateRange + " is not a valid range");
+		}
+		this.dateRange = dateRange;
 	}
+
 }