Tool in UNIX to subtract dates

Is there any tool in Solaris UNIX (so no GNU tool available) to subtract dates? I know that in Linux we have gawk that can subtract one date from another. But in Solaris the maximum we have is nawk (improved awk) which cannot perform date calculations. Also I cannot use perl.

Text between two tags

I want to retrieve whatever is between these two tags – <tr> </tr> – from an html doc.
Now I don’t have any specific html requirements that would warrant for an html parser. I just plain need something that matches <tr> and </tr> and gets everything in between and there could be multiple trs.
I tried awk, which works, but for some reason it ends up giving me duplicates of each row extracted.