On an OpenBSD system:
$ stat -f '%Sm' myfile
Apr 12 09:40:05 2018
The %Sm format means "modification time (m), as a string (S)".
Without the S, you'll get a Unix timestamp:
$ stat -f '%m' myfile
1523518805
These are the equivalents of %y and %Y with GNU stat -c "FORMAT" apart from the slight difference in the %y format output.