One of the utilities that I immediately install on every new Mac is TextExpander. It makes typing routine stuff so much easier.
In a recent blog post, David Sparks wrote about his TextExpander snippets for date and time, describing two simple but effective use cases: Shortcuts like "xm8", which expands to "August" (why haven't I thought of this myself?), and date calculation snippets like "d--" that inserts yesterday's date.
Using David's snippets on your Mac requires only three mouse clicks (on the link above, on the download link in his post and finally on the snippet file).