Datetime operations with gmtmath

Q&A
1

Salut everyone! Could you please let me know how I can, for example, add one day to the date time format using the math function keeping datetime format. The line below does not work:

gmt math -Q 2021-10-05T05:05:05 1 ADD --TIME_UNIT=d =

The desired output is 2021-10-06T05:05:05

2

What’s the output you got? (Maybe add -fT)

3

gmt math -Q -fT 2021-10-05T05:05:05 1 ADD --TIME_UNIT=d =

4

Beautiful! thanks a lot!

5 
gmt math -Q -fT 2021-10-05T05:05:05 1 ADD --TIME_UNIT=d =
2021-10-06T05:05:04

Why does this command change the time from 05:05:05 to 05:05:04?

6

I noticed that too. If I do

gmt math -Q -fT 2021-10-05T05:05:05.00001 1 ADD --TIME_UNIT=d =

then it rounds up to 5, so it is some sort of roundoff since internally every time is a double precision days since the epoch which is at 1970. So undoing the formatting and just getting the days

gmt math -Q 2021-10-05T05:05:05 1 ADD --TIME_UNIT=d =
18906.2118634
gmt math -Q 2021-10-05T05:05:05 0 ADD --TIME_UNIT=d =
18905.2118634

they are 1 apart, but clearly breaking this down to hh:mm:ss suffers from roundoff…
Will have to debug at some point.

1 Like
7

Interesting. Thanks for the insight.

8

And if you do the math with seconds (86400) and no UNIT setting it works fine due to the smaller granularity of seconds vs days.

1 Like