Strange date behaviour in bash

Question

I am trying to add hours to a given datetime in bash and its giving weird results

This is just to check if its parsing the date time correctly

$ date -d "2020-01-21 12:00:00"

Tue 21 Jan 12:00:00 UTC 2020

Now when I try to do date math

$ date -d "2020-01-21 12:00:00 +2 hour"

Tue 21 Jan 11:00:00 UTC 2020

I tried few other operations but similar behaviour.

If I change my format then it behaves correctly for eg.

date -d "12:00:00 2020-01-21 +2 hour"

Tue 21 Jan 14:00:00 UTC 2020

not sure whats going on here.

Answer 1

The +2 is being interpreted as an explicit timezone specifier (equivalent to EET), so date -d "2020-01-21 12:00:00 +2 hour" is interpreted the same as date -d "2020-01-21 12:00:00 EET hour" , which adds one hour to the timezone-adjusted time specified.

You can either provide an explicit timezone (as suggested by Maaz) so that +2 hour is syntactically an additional offset, or you can move it to the beginning of the expression, where it can't be parsed as a timezone.

date -d "+ 2 hour 2020-01-21 12:00:00"

Answer 2

Basically you need to pass the timezone in command as well like UTC/CT/IST, so that date command can understand the point of reference to add extra hours.

For example if you are in UTC timezone, then following will produce correct output for you.

date -d "2020-01-21 12:00:00 UTC + 2 hour"