Since version 1.21.0 (released very soon) 3G Watchdog Pro allows you to specifiy a Time Zone for your local/domestic and roaming plan (in 3GW Settings > Your Plan). This is the reference Time Zone for the dates used by your operator (billing date, day, week, month). It defaults to your current TZ at the time this parameter is first set. Note that the chosen TZ applies to both the local and the roaming plans (i.e. the two plans share the same TZ).
The interest of specifying a Time Zone is that if you happen to travel and change Time Zone, 3G Watchdog will still be able to calculate correctly the dates using the appropriate reference TZ. When you are in a different TZ than the plan’s one 3GW will display the date in the plan TZ at the top of the section (local or roaming) for this plan in the usage report (main) screen. This will remind you that the date is different from the date in your current TZ.