Find

Between dates

find / -newermt "27 Dec 2013" ! -newermt "1 Jan 2014"

-newermt newer than specified modification time
! invert proceeding condition

Several matches

find / -xdev \( -name .DS_Store -o -name ._.DS_Store \)

-xdev stay on the current filesystem
-o logical OR, use parentheses for clarity - must be escaped in bash

Find files and do something with each

find . -type f -print0 | xargs -0 <command>

-print0 print the full file name on the standard output, followed by a null character (instead of the newline character that -print uses)
-0 input items are terminated by a null character instead of by whitespace, and the quotes and backslash are not special (every character is taken literally).