It is just a note, as I figured out the solution consulting the documentation. When calculating the duration between two time stamps, I get
gmt math -Q 2021-02-05T19:22:51 2021-02-05T18:40:11 SUB =
2560
So, the interval is given in seconds. If I want to do the same with two files containing the respective time stamps, I need an extra flag, -fot .
echo '2021-02-05T19:22:51' > 1.dat
echo '2021-02-05T18:40:11' > 2.dat
gmt math -fot 1.dat 2.dat SUB =
2560
Without it I get the time difference in absolute time:
1970-01-01T00:42:40
So, the default in quick mode seems to be -fot whereas otherwise it is -foT.