The validity period of your data plan starts at the time you first use it in your destination country and expires after the number of days shown for the plan you purchased, e.g. 14 days. For example, if you start using your 30-day plan at 10.00 am local time, it will expire at 10.00 am local time, thirty days later.
It is also very important to keep in mind that data plans will expire before the expiration date if all of the plan's data balance is used before the expiration date. If this happens, you will need to top-up your plan to continue using the service.