Time.org.uk

In the UK the clocks change to British Summer Time (BST) and Greenwich Mean Time (GMT) at 01:00 hours GMT on the last Sunday of March and October respectively.

When the change to BST occurs your clock will show 01:00 and so you should move it forward one hour to 02:00 making clock time appear to progress as...

00:59:58
00:59:59
02:00:00
02:00:01

...which looks like 01:00:00 to 01:59:59 just doesn’t occur!

Moving from GMT to BST has the effect of losing one hour of sleep for most people.

When the change to GMT occurs your clock will show 02:00 and so you should move it backward one hour to 01:00 making clock time appear to progress as...

01:59:58
01:59:59
01:00:00
01:00:01

...which looks like 01:00:00 to 01:59:59 occur twice that night but 02:00:00 just once!

Moving from BST to GMT has the effect of gaining one hour of sleep, or an extra hour awake, for most people.