truetype/HeadTable: Make TrueType date calculations UTC-based

The TrueType epoch is supposed to be understood as an UTC date.
Instantiating `GregorianCalendar` with a date implicitly sets its
timezone to the local timezone.

Because it is not possible to pass both a date and a timezone to the
constructor of `GregorianCalendar`, let's store the offset as a
precalculated number (`date --date="1904-01-01 00:00:00 UTC" +%s`).

While we are at it, let's also switch from negative- to
positive-offset calculations.

Author: gioele

main/java/BitsNPicas/src/com/kreative/bitsnpicas/truetype/HeadTable.java

@@ -24,7 +24,7 @@ public class HeadTable extends TrueTypeTable {
 	public static final int FLAGS_DEFINED_BY_ADOBE_2                    = 0x1000;
 	public static final int FLAGS_DEFINED_BY_ADOBE_3                    = 0x2000;
 	public static final int FLAGS_GENERIC_SYMBOLS_FOR_CODE_POINT_RANGES = 0x4000;
-	public static final long DATE_EPOCH = new GregorianCalendar(1904, Calendar.JANUARY, 1, 0, 0, 0).getTimeInMillis();
+	public static final long DATE_EPOCH_1904 = 2082844800L * 1000L; // Milliseconds between 1904-01-01 00:00:00 UTC and Unix epoch
 	public static final int MAC_STYLE_PLAIN       = 0x00;
 	public static final int MAC_STYLE_BOLD        = 0x01;
 	public static final int MAC_STYLE_ITALIC      = 0x02;
@@ -81,23 +81,23 @@ public void setFontRevisionDouble(double revision) {
 	}
 
 	public GregorianCalendar getDateCreatedCalendar() {
-		GregorianCalendar cal = new GregorianCalendar();
-		cal.setTimeInMillis(DATE_EPOCH + (dateCreated * 1000L));
+		GregorianCalendar cal = new GregorianCalendar(TimeZone.getTimeZone("UTC"));
+		cal.setTimeInMillis((dateCreated * 1000L) - DATE_EPOCH_1904);
 		return cal;
 	}
 
 	public void setDateCreatedCalendar(Calendar cal) {
-		dateCreated = (cal.getTimeInMillis() - DATE_EPOCH) / 1000L;
+		dateCreated = (cal.getTimeInMillis() + DATE_EPOCH_1904) / 1000L;
 	}
 
 	public GregorianCalendar getDateModifiedCalendar() {
-		GregorianCalendar cal = new GregorianCalendar();
-		cal.setTimeInMillis(DATE_EPOCH + (dateModified * 1000L));
+		GregorianCalendar cal = new GregorianCalendar(TimeZone.getTimeZone("UTC"));
+		cal.setTimeInMillis((dateModified * 1000L) - DATE_EPOCH_1904);
 		return cal;
 	}
 
 	public void setDateModifiedCalendar(Calendar cal) {
-		dateModified = (cal.getTimeInMillis() - DATE_EPOCH) / 1000L;
+		dateModified = (cal.getTimeInMillis() + DATE_EPOCH_1904) / 1000L;
 	}
 
 	@Override