is not a grep option what you are looking for, but a match to use inside grep

If you want to match only lines with 03-15-2021 followed by a space (so not a dash) you can use the regex character class dedicated to space-like stuff. See it in perlrequick#Using-character-classes

L*

PS you can practice online using marvellous webperl (already filled with your example, modify the regex with what already suggested) and you can can find other regex practicing tools in my bibliotheca