OS X uses FreeBSD date which has totally different cmdline options. I did actually write an awk script that works on Centos 6, I'll upload that shortly. (Edit: I think instead of -jf, you can use --date= on linux.)
The event suggestion is good. Really I just threw the script together last month and occasionally fixed it up when I showed it to friends, so maybe I'll take some time to add that and fix some other things about this (like a prettier graph ^) when I finish my last stretch of schoolwork in the next few weeks. But in the mean time feel free to add that if you want :)
