Our world(Earth) is devided into many timezones. Means at a moment we have different different times in different regions.
So each region is given a timezone
We have considered a region as idle and according to it, other regions are given the timezones. The idle region has timezone : GMT
In web application and mobile application, the developer should give the time to user in user’s region’s timezone.
Find your best matched combination and use it. here you have timezone select box in html