Added printTimestamp function in "HH:MM" format

Testat-Planer.zig

@@ -37,7 +37,7 @@ pub fn main () !void {
 
 // Printet timestamp im Format HH:MM
 pub fn printTimestamp(ts: *Timestamp) !void {
-    try writer.print("{d}:{d}", ts.*);
+try writer.print("{0d:0>2}:{1d:0>2}", .{ ts.hours, ts.minutes });
 }
 
 pub fn printPlan() !void {