What is Daylight Saving Time?
Daylight Saving Time (DST) is the practice of moving clocks one hour forward in spring and then one hour back in autumn.
Here is how it works:
In spring, clocks go forward one hour at a particular date and time (varies by country). For example, in the UK, the clock goes forward on the last Sunday in March at 1:00AM GMT so that the time changes instantly from 1:00AM GMT to 2:00AM BST.
In autumn, clocks go back an hour at a particular date and time (varies by country). For example in the UK, the clock goes backward on the last Sunday in October at 2:00AM BST so that the time changes instantly from 2:00AM BST to 1:00AM GMT.
The time change happens in the early hours of the morning to minimise disruptions to work or school routines.
Not all countries observe Daylight Saving Time but here are the dates and times for some that do:
| Region | DST Starts | DST Ends | Local Change Time |
|---|---|---|---|
| Europe / UK | Last Sun in March → 30 Mar 2025 | Last Sun in Oct → 26 Oct 2025 | 01:00 UTC (affects local time) |
| USA / Canada | 2nd Sun in March → 9 Mar | 1st Sun in Nov → 2 Nov | 02:00 local time |
| Cuba | 2nd Sun in March → 9 Mar | 1st Sun in Nov | 00:00 forward / 01:00 back |
| Egypt | Last Fri Apr → ~25 Apr | Last Thu Oct → ~30 Oct | Midnight shifts |
| Chile (continental) | 1st Sat Sep → ~6 Sep | 1st Sat Apr | 24:00 jumps |
| Australia (selected) | 1st Sun Oct → ~5 Oct | 1st Sun Apr | 02:00 → 03:00 |
| New Zealand | Last Sun Sep → ~28 Sep | 1st Sun Apr | 02:00 → 03:00, etc. |
Why does it exist?
TODO
How does it affect software?
TODO