date/time can only changed per system wide. However, you may also do the change by using some of the date/time function to reflect the date/time you wish to shows up in your domain's application.
Yes, ts is right. You must change the server time system wide, as the server really only resides in one area of the world, although I realize your customers may be all throughout the world and this is unfortunate.
Since 99% of my customers are here in Los Angeles, I've set the time in accordance with that (Pacific Time.)