The curious case of the phantom dates

Recently I had to fix a bug where a DateFormatter would return nil when trying to convert a String to a Date. The date string was in a perfectly valid format and the formatter would return nil while on two dates when all the other dates could be transformed without problem.

Here is a screenshot of a playground that I used to examine the problem:

As you can see the formatter does not have a problem with 2022-03-27T01:30:00 but for some reason it cannot transform 2022-03-27T02:00:00 to a valid date

I observed that this invalid dates only occur on March 27th between 02:00a.m. and 02:59a.m.

And then it dawned on me. Daylight Saving Time! Of course. In my country we change our clocks on March 27th directly from 01:59 to 03:00. So the formatter is correctly returning nil when trying to format a date that lies in the hour that does not exist during that night.

So a DateFormatter is quite clever and knows about the oddities of DST. Turns out, that DateFormatter is even more clever when you want it to be: By setting its isLenient property to true the formatter uses heuristics to guess what date you wanted when you provides the date string for a date that does not exist.

Here is what happens when you let the DateFormatter guess what you really wanted:

As you can see now the formatter correctly transforms the 02:00a.m. date to 03:00.a.m. during the night of the switch to Daylight Saving Time.

This might not always be what you want, but in my case it was the (easy) fix to the bug.