By default, all metrics in reports aggregated using Moscow, Russia timezone. However, you could change this using Timezone report setting and use any other timezone.
This will affect report, if any date/time dimension (Hour, Date, Week day, etc.) used for aggregation.
This could be useful if you need to compare metrics from different systems using different time zones, or if you application distributed in area uses time zone other than the default.
You can set up Timezone for all MyTracker reports at once. For more details, refer to Settings section.