Datetime.newInstance and Datetime.newInstanceGmt are not working correctly

Question

My user's timezone is GMT+2 (Eastern European Time) and I notice strange thing when playing around with dates.

Date myDate = Date.newInstance(2019, 11, 29);
Time myTime = Time.newInstance(9,0,0,0);

System.debug(Datetime.newInstance(myDate, myTime));
System.debug(Datetime.newInstanceGmt(myDate, myTime));

I am creating Datetime instance from 9 hour. I expect Datetime.newInstance to return Datetime with 9 o'clock. And for newInstanceGmt I expect it to be 7 o'clock, because when in my timezone it's 9, in London it's 7 am.

But the result I get is the following:

09:44:51.11 (12596972)|USER_DEBUG|[4]|DEBUG|2019-11-29 07:00:00
09:44:51.11 (12638407)|USER_DEBUG|[5]|DEBUG|2019-11-29 09:00:00

It's flipped over. For some reason Salesforce is instantiating newInstance with subtracting my timezone offset, which is not logical to me.

Answer 1

System.debug(dateTimeVariable) always outputs a DateTime value in GMT. What you're looking at is 7:00:00 AM GMT and 9:00:00 AM GMT, respectively. To get the results you expect, use format() instead.

Date myDate = Date.newInstance(2019, 11, 29);
Time myTime = Time.newInstance(9,0,0,0);

System.debug(Datetime.newInstance(myDate, myTime).format());
System.debug(Datetime.newInstanceGmt(myDate, myTime).format());