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] = 02:59 BST, 29 June 2017 (displays current date and time)
[time=5pm 4 Feb] = 17:00 GMT, 4 February 2017
[time=2004-09-05 19:00 GMT] = 20:00 BST, 5 September 2004
[time=2004-09-05 19:00:00] = 20:00 BST, 5 September 2004
[time=+1 week 2 days 4 hours 2 seconds] = 06:59 BST, 8 July 2017
[time=+1 week] = 02:59 BST, 6 July 2017
[time=+1 day] = 02:59 BST, 30 June 2017
[time=15:30 GMT 16 May 2004] = 16:30 BST, 16 May 2004
[shorttime=now] = 02:59 BST (displays current time)
[shorttime=19:00 BST] = 19:00 BST
[time=next Thursday] = 01:00 BST, 6 July 2017
[time=next Thursday 19:00 BST] = 19:00 BST, 6 July 2017
[time=last Monday] = 01:00 BST, 26 June 2017
[time=last Monday 19:00 BST] = 19:00 BST, 26 June 2017
[time=yesterday] = 01:00 BST, 28 June 2017
[time=yesterday 19:00 BST] = 19:00 BST, 28 June 2017
[time=tomorrow] = 01:00 BST, 30 June 2017
[time=tomorrow 19:00 BST] = 19:00 BST, 30 June 2017

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