I am scared to do this but... Shouldn't it be " Those were " instead of "That was" ??
While technically accurate (and God knows I can nit-pick about language usage), the term "good times," as used in the common vernacular, can be used to refer to a condition or state of mind/being in the singular adjective sense, and not in reference to "times" in the plural form. Syntax aside, it just sounds cool.

Brett