ConkyDateDiff and conkyDaysDiff (en)
From Conky PitStop
conkyDateDiff.py and conkyDaysDiff.py
Put together here as both are used to create the conky seen below.
Here's another couple of python apps by Kaivalagi. I emailed him because it came to light that HowLong.pl was displaying errors well this works just fine - to the day in fact.
The conkyrc, conkyDateDiff.py and conkyDaysDiff.py are in the archive : tar.gz
NOTE : conkyDateDiff.py depends on python-dateutil
Check out the accuracy of these at timeanddate.com