You can use "3 months ago" directly with the "newerXY" syntax:
find . -maxdepth 2 -xdev -type f -newermt '3 months ago'
(I added maxdepth and xdev to limit the search)
With these relative dates minutes, hours, days, weeks and months can be used.
"m" stands for modification time, "t" means direct time (and not the date of a reference file).
Or by combining two absolute dates to choose a time span; here from Jan 1 to Jan 15:
#] find . -maxdepth 2 -newermt 'Jan 1' ! -newermt 'Jan 15' -ls 415640 4 -rw-r--r-- 1 root root 2190 Jan 8 15:14 ./df.man 412465 4 -rw-r--r-- 1 root root 98 Jan 1 2020 ./ranfunc 412282 4 -rw------- 1 root root 23 Jan 13 16:53 ./.python_history 406542 0 lrwxrwxrwx 1 root root 56 Jan 8 19:46 ./.fvwm/.BGdefault -> /usr/share/fvwm/default-config/images/background/bg3.png 417900 28 -rw-r--r-- 1 root root 25990 Jan 3 09:48 ./xterm.2020.01.03.09.48.03.xhtml