How can I configure git log to show commit date instead of author date?
There are several options to pretty print the date. Probably the easiest is to just use one of the pre-baked --pretty formats, like git log --pretty=fuller - this will show both dates. If you want to see only one date, but make it the commit date, you can use git log --format=<some stuff>. All the allowable codes for defining the format are documented in git help log. The commit date is one of %cd, %cD, %cr, %ct or %ci, depending on what format you prefer it in.
If it's something you want to do often, put it in an alias or write an auxiliary script to save on typing.