[sischoel]

It might be possible that candidates are afraid that their solution is not clever enough for you, i.e they assume that you want something like a one line formula that only contains numbers and basic arithmetic operators.

The way I would solve something like that is by mentioning that it is a good approach to do time calculations with some library functions so that I won't have to deal with all the weird edge cases, then start with a loop over the months and later refine that to get rid of the loop, but it is very much possible that an interviewer might view this loop negatively.

There might also be some cultural aspects. While the Gregorian calendar is probably somehow used everywhere in the world, I think there might also be some other calendar formats being used in parallel.

If it is the first interview of the day, maybe the candidates are also just nervous? I remember when I had my on site interview at Google, the first warm up question had something to do with probability and could be easily solved with linearity of expectation. In the months before that interview I had dealt quite a lot with probability for my master thesis - but at that day I was quite nervous and I had not slept well, so I spend quite a lot of time on that question. I think I answered later questions better, although it was not enough in the end.

Do you usually see some correlation between the warm-up question and later performance?

I am not sure if that is necessary, but what might help is by starting to calculate the number of days from the start of the month and then adapt the question.

[faanghacker]

I've gotten some candidates who come from backgrounds with non gregorian calendars. In those cases we spend a minute talking about their calendar and then I ask them to write it for their calendar.