Solaris’ built-in date utility is very primitive and does not support many of the things I use `date` for. In the past when I had a need for more modern facilities, I used to install `coreutils` from sunfreeware.com.

Unfortunately they recently changed their policy and no longer offer free (gratis) downloads of GNU open source tools. I have no knowledge of an alternative solution, but if you do find one – it would be great if you can post it here so that other people can also get help 🙂