This shows how date ticks and labels may be generated according to local
time (default) or UTC with the option
72 hours of random hourly data since 2009-Jul-23 18:00 UTC according to local time (top) and UTC (bottom):
Please note the offset between the respective ticks in both plots. It should match your local time zone offset.
You can also check it by hovering over corresponding points and comparing the value labels.
Try different zoom levels to show that ticks are always placed at nice time boundaries.