Datetime.addDays method works unexpected with daylight saving time offset day

Question

I have a small piece of code which is displaying the Day of the week which is part of a larger method.

Date StartDate = Date.newInstance(2018, 11, 4);
Datetime StartDateDt = Datetime.newinstance(StartDate.year(),StartDate.month(),StartDate.day());
System.debug(StartDateDt);
System.debug(StartDateDt.format('EEEE') );
StartDateDt = StartDateDt.addDays(1);
System.debug(StartDateDt);
System.debug(StartDateDt.format('EEEE') );

If you look at the debug on the dev console

Any thoughts on what is happening here? Why are both Nov 4 and Nov 5 2018 showing as Sunday?

Update

if i was using GMT like what @Oleksandr Berehovskiy suggested then i get 4 nov as Saturday and 5 Nov as Sunday. Actually 4th is Sunday and 5th is a Monday

What i wanted was if i was running the code from the users locale then we should get the weekday from user's locale and not GMT

Interestingly the issue is only if the start date is given as 4th Nov. If i give the start date as 5 nov then it works fine

Answer 1

The reason of such behavior is daylight saving time offset.

You are doing everything okay. Result is expected with Datetime class. Your user's timezone is America/Chicago. According to Daylight Saving Time Changes 2018 article, 4-th of November was a day, when clock went back 1 hour.

Sunday, 4 November 2018, 02:00:00 clocks were turned backward 1 hour to

Sunday, 4 November 2018, 01:00:00 local standard time instead.

I have simplified your code to the following.

Datetime myDateTime = Datetime.newInstance(2018, 11, 4);

//DEBUG|myDateTime.format(yyyy-MM-dd HH:mm:ss):2018-11-04 00:00:00
System.debug('myDateTime.format(yyyy-MM-dd HH:mm:ss):' + myDateTime.format('yyyy-MM-dd HH:mm:ss'));

//DEBUG|myDateTime.formatGMT(yyyy-MM-dd HH:mm:ss):2018-11-04 05:00:00
System.debug('myDateTime.formatGMT(yyyy-MM-dd HH:mm:ss):' + myDateTime.formatGMT('yyyy-MM-dd HH:mm:ss'));

Datetime newDateTime = myDateTime.addDays(1);

//DEBUG|newDateTime.format(yyyy-MM-dd HH:mm:ss):2018-11-04 23:00:00
System.debug('newDateTime.format(yyyy-MM-dd HH:mm:ss):' + newDateTime.format('yyyy-MM-dd HH:mm:ss'));

//|DEBUG|newDateTime.formatGMT(yyyy-MM-dd HH:mm:ss):2018-11-05 05:00:00
System.debug('newDateTime.formatGMT(yyyy-MM-dd HH:mm:ss):' + newDateTime.formatGMT('yyyy-MM-dd HH:mm:ss'));

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Datetime myDateTime = Datetime.newInstance(2018, 11, 4);

here you constructing local Chicago time 4-th of November 00:00:00

after 2 hours, it is 02:00:00 and immediately time goes back 1 hour and it becomes 01:00:00 time. So you have not 24 hours per day, but 25.

Looks like Datetime.addDays(1) method adds 24 hours to Datetime object and that is why after myDateTime.addDays(1) you have 2018-11-04 23:00:00. If you add 25 hours to it, you will get expected 2018-11-05 00:00:00 and Monday result.

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What final conclusion I did from the current case.

If you don't have to work with/get/manipulate with hours, minutes and seconds - always use Date class. Because it adds days correctly with all daylight saving time offsets.

Date myDate = Date.newInstance(2018, 11, 4);

//DEBUG|myDate.format(yyyy-MM-dd):11/4/2018
System.debug('myDate.format(yyyy-MM-dd):' + myDate.format());

Date newMyDate = myDate.addDays(1);

//DEBUG|newMyDate.format:11/5/2018
System.debug('newMyDate.format:' + newMyDate.format());
DateTime newMyDateTime = DateTime.newInstance(newMyDate.year(), newMyDate.month(), newMyDate.day());

//newMyDateTime.format(yyyy-MM-dd HH:mm:ss):2018-11-05 00:00:00
System.debug('newMyDateTime.format(yyyy-MM-dd HH:mm:ss):' + newMyDateTime.format('yyyy-MM-dd HH:mm:ss'));

//DEBUG|newMyDateTime.format(EEEE):Monday
System.debug('newMyDateTime.format(EEEE):' + newMyDateTime.format('EEEE'));

from upper snippet, we convert Date to Datetime at the latest step, where we need to know what is a day of the week. But all manipulations we did with Date class