Simple math

It takes a few minutes to do the math, a simple calculation to determine the number of years we have been married.  But I need to stop and think about it.  My wife just knows.  It's the same with birthdays and other celebrations that need to be memorialised.  I need a written prompt but it seems to have been enoded into her DNA.  It must just be another one of those mysteries of married life, although I can't say which one - I've lost count.