How can I tell if DateTime.Now() is on a day AFTER a different DateTime

Question

I'm running this on flutter, but I guess this could be a more general issue.

I am saving a DateTime in the preferences. I want to be able to then tell if DateTime.now() is on at least a day after the last saved DateTime, i.e.

(pseudocode)
lastDailyCheck = 2020.04.10
now = 2020.04.11

=> now is a day after the lastDailyCheck.

This should already work if it is 00:01 on the new day, even if the lastDailyCheck was on 23:58 the day before, meaning the difference can be as low as minutes between the 2 DateTimes.

Turns out this is really complicated!

Here's what doesn't work:

DateTime.Now().isAfter(lastDailyCheck)

This only checks if now is after the last one, it also return true after a second, and on the same day.

DateTime.Now().isAfter(lastDailyCheck) && lastDailyCheck.day != DateTime.Now().day

I thought this was clever. If the day is different and it is after the last then it does work in recognizing it is a day later - but then I realized it would bug out when both days are say on the 15th of the month - then lastDailyCheck.day would equal DateTime.Now().day.

What do you think would be possible here?

Answer 1

I don't know flutter, but my approach would be to not store the last check, but store the date at which the next check should occur. So when you perform a check you calculate the next midnight and store that. Now you can use isAfter.

In javascript this would look something like this:

const now = new Date();

//this also handles overflow into the next month
const nextCheck = new Date(now.getFullYear(), now.getMonth(), now.getDate() + 1)

//store nextCheck somewhere

//in js there is no isAfter, you just use >
if(new Date() > nextCheck) {
   //do the thing
}

of course you could also calculate nextCheck every time you want to compare it, but I dislike performing the same calculation over and over if I can avoid it.

A thing to mention here is timezones, depending on your date library and if your system and user timezones align, you may need to shift the date.