Tonight, in Montreal (and many other North American cities), we change from Standard Time to Daylight Time.

I know we programmers complain about date/time math relentlessly, but I thought it was worth sharing this real-life problem that someone asked me about on Reddit this weekend:

It sounds like this is a serious problem that has effected [sic] you on more than one occasion. Story?

The simplest complicated scenario is: let's say we have a call scheduled between our team on the east coast of North America and a colleague in the UK at 10AM Montreal time.

Normally Nottingham (UK, same as London time) is 5 hours ahead of Montreal. This is pretty easy. Our British colleague needs to join at 3PM.

However, tonight, we change from EST to EDT in Montreal (clocks move one hour ahead). But the UK will still be on GMT tomorrow. So, now, the daily 10AM call becomes a 2PM call for the Brits.

But this is only for the next 2 weeks, because BST starts on March 26th (BST is to GMT as EDT is to EST). Then, we go back to a 5 hour difference. So we can expect Europeans to show up an hour late for everything this week. Or maybe we're just an hour early on this side of the Atlantic.

To make this more difficult, we often have calls between not only Montreal and England, but also those two plus Korea and Brazil.

Korea doesn't employ Daylight Saving Time, so a standing 7AM call in Seoul (5PM in Montreal) becomes a 6PM call in Montreal.

And to even further complicate things, our partners in São Paulo switched FROM DST to standard time on Feb 17. Because they're in the southern hemisphere the clock change is the opposite direction of ours, on a different day.

So: yes. It has affected our team on many occasions. It's already very difficult to get that many international parties synced up. DST can make it nearly impossible.