There's a lot of room for error there in that not every year has 365 days, and not every month has 30 days!
You might get more accurate results if you convert your dates to epoch time, do the math with those numbers, and then convert the result back to something easier to read. Here's a starting point:
# Current time/date in seconds...
# Midnight on Jan 1st, 2000
date -d "01/01/2000" +%s
# Then we can subtract those numbers, and convert
# the result to days (if that's what you're looking for)
century=$(date -d "01/01/2000 00:00:00" +%s)
Disclaimer: not a lot of thought went into this demonstration, so I can vouch for the accuracy, but I think it's close, and should get you going in the right direction!
Hope this helps!