Clocks go forward tonight
The clocks go forward one hour at midnight tonight - the last Saturday in March.
This means the UK will be one hour ahead of GMT or Greenwich Mean Time. GMT was adopted by the railways in the 1840s, as timetables required every station to be using the same time system. Prior to the railways, time was based much more around the sun and because no-one could travel very far in a day, it didn't cause any issues.
GMT became the British standard in 1880 and the USA very quickly also adopted it. GMT became the global standard in 1884.
Changing the clocks in summer and back in winter was adopted in the UK in 1916, as an energy saving measure during the 1st world war. There is ongoing discussion about making the change permanent, in which case GMT would fall out of use.