Last Updated: 2024-11-21

Summer time / "Daylight Savings Time" basics

• Depends on EU state whether it is applied. Soon not in Germany (but yes in other states)
• EU: Clocks add an hour on the last sunday of March (not a fixed day, like 31st, as I once thought). Then the clocks go back one hour on the last Sunday of October.

How to tell them when summer time is applying?

Solution 1: They will have different TimezoneOffsets

startDate.getTimezoneOffset()
-60
endDate.getTimezoneOffset()
-120

Solution 2: When logged, one will say Standard Time and the other will say Summer Time.

console.log(startDate)
=> Wed Nov 05 2008 00:00:00 GMT+0100 (Central European Standard Time)

console.log(endDate)
// notice how the Summer/Standard bit is only at the end of the input...
// be sure to read the full thing!
=> Wed Jul 01 2009 00:00:00 GMT+0200 (Central European Summer Time)

Why was the TimezoneOffset negative?

I am writing from Germany, GMT+1 (not summer time). That means 2pm here corresponds to 1pm in the UK. So, from my perspective, the amount I would need to modify the current time by - i.e. the offset - would be -60 minutes in standard time (and -120 minutes in summer time). The baseline is not whether there is also daylight savings time in the UK - but rather a platonic unmoving time center.