Below is a list of available timetags you can use in news posts. The time shown on the screen is converted to the person's local time as decided by their timezone setting.

[time=now] = 06:12 CST, 22 January 2017 (displays current date and time)
[time=5pm 4 Feb] = 11:00 CST, 4 February 2017
[time=2004-09-05 19:00 GMT] = 14:00 CDT, 5 September 2004
[time=2004-09-05 19:00:00] = 14:00 CDT, 5 September 2004
[time=+1 week 2 days 4 hours 2 seconds] = 10:13 CST, 31 January 2017
[time=+1 week] = 06:12 CST, 29 January 2017
[time=+1 day] = 06:12 CST, 23 January 2017
[time=15:30 GMT 16 May 2004] = 10:30 CDT, 16 May 2004
[shorttime=now] = 06:12 CST (displays current time)
[shorttime=19:00 BST] = 12:00 CST
[time=next Thursday] = 18:00 CST, 25 January 2017
[time=next Thursday 19:00 BST] = 12:00 CST, 26 January 2017
[time=last Monday] = 18:00 CST, 15 January 2017
[time=last Monday 19:00 BST] = 12:00 CST, 16 January 2017
[time=yesterday] = 18:00 CST, 20 January 2017
[time=yesterday 19:00 BST] = 12:00 CST, 21 January 2017
[time=tomorrow] = 18:00 CST, 22 January 2017
[time=tomorrow 19:00 BST] = 12:00 CST, 23 January 2017

Note that you do not have to provide times in GMT, it is converted later to local times by the site automatically.