Dealing with date times is one of the hardest problem a software engineer deals with.
Many of us wish every place in the world used UTC: no timezone, no summer/winter time. It would be a better place haha.

I’ve come across a problem recently that made me think about what exactly happened when the clock changes.
This post is a collection of thoughts about date time changes.

Observations

Winter to Summer time in the UK

On 31 March 2019 at 1am in the UK the clock jumped to 2am.
Does it mean 31 March 2019 01:30am doesn’t exist? 🤔

I live in London, if I had planned to meet somebody on March 31 01:30am London time, that would have been a problem!

It got me thinking, what happens in JavaScript when creating a Date object around this date time.

WARNING: I live and run my code IN LONDON. And today is 10 November.
London timezone is GMT (Greenwich Median Time) during Winter which is equal to UTC
But it’s BST (British Summery Time) during Summer which is 1 hour ahead of UTC: UTC+1.

This is what I get:

$ new Date(2019, 2, 31, 0, 30, 0):  2019-03-31T00:30:00.000Z
$ new Date(2019, 2, 31, 1, 30, 0):  2019-03-31T01:30:00.000Z
$ new Date(2019, 2, 31, 2, 30, 0):  2019-03-31T01:30:00.000Z
$ new Date(2019, 2, 31, 3, 30, 0):  2019-03-31T02:30:00.000Z
$ new Date(2019, 2, 31, 4, 30, 0):  2019-03-31T03:30:00.000Z

WARNING: Months are 0 indexed.

31 March 2019 00:30am is converted to 31 March 2019 00:30am UTC. It makes sense as at this time, the UK is still on GMT so follows UTC.
31 March 2019 02:30am is converted to 31 March 2019 01:30am UTC. It makes sense as at this time, the UK is on BST, i.e. UTC+1.
Now, as you can see, 31 March 2019 01:30am which actually doesn’t exist since the clock jumps, doesn’t fail and exist. It’s considered Winter time.
Should NodeJs be smart enough and fail? Is this behaviour defined somewhere? I honestly don’t know yet. To be continued… 🕵️‍

Summer to Winter time in the UK

On 27 October 2019 at 2am in the UK the clock went backwards to 2am.
Wait, did we re-live the same hour twice? Does it mean 27 October 2019 01:30am happened twice? 🤔

Again, I live in London, if I had planned to meet somebody on March 27 01:30am London time, which 01:30am would that have meant?!

I also looked at what happens in JavaScript around this date time:

$ new Date(2019, 9, 27, 0, 30, 0):  2019-10-26T23:30:00.000Z
$ new Date(2019, 9, 27, 1, 30, 0):  2019-10-27T00:30:00.000Z
$ new Date(2019, 9, 27, 2, 30, 0):  2019-10-27T02:30:00.000Z
$ new Date(2019, 9, 27, 3, 30, 0):  2019-10-27T03:30:00.000Z
$ new Date(2019, 9, 27, 4, 30, 0):  2019-10-27T04:30:00.000Z

27 October 2019 00:30am is converted to 26 October 2019 23:30am UTC. It makes sense as at this time, the UK is still on BST, i.e. UTC+1.
27 October 2019 02:30am is converted to 27 October 2019 02:30am UTC. It makes sense as at this time, the UK is back on GMT and follows UTC.
Now, as you can see, 27 October 2019 01:30am is converted to 27 October 2019 00:30am UTC. It makes sense because the clock only changes at 2am BST. HOWEVER, after the clock has gone backwards, 27 October 2019 01:30am should then be converted to 27 October 2019 01:30am UTC.

If we just look at the UTC date times, it looks like there’s a gap. There’s nothing between 00:30am and 02:30am. It’s because when one says 27 October 01:30am London time, it’s ambiguous! Without knowing whether one is talking about BST or GMT, one can’t know what time is exactly referred to.
Should nodeJs be smart enough and fail? Complaining it’s ambiguous? I don’t know either. To be continued… 🕵️‍

So, I have a few things to think about and investigate.
What’s for sure is that time zones and daylight savings are really hard.
But I’m also quite worried about what’s going to happen when we get rid of it in Europe (deadline 2021). I’m worried about the legacy system with “hard-coded bug fixes” that cater for DST. I need to think about this more though!