Ensure consistent firstTime for all timezones.

The LineChartTimeTicker test fails in locations with a nonzero minute timezone offset e.g. GMT + 05:30(India) as the ticks generated by LineChartTimeTicker for the 6-hour span will have a 30 min offset from the expected ticks. This patch ensures the ticks are consistent across all timezones to make the tests pass in all timezones