I've seen that number in a lot of msl scripts. The solution I found was:
Days in a year = 365.2425 -->> This number is explained here -->> http://en.wikipedia.org/wiki/Gregorian_calendar#Description
Seconds in a day = 86400
$calc(365.2425 * 86400) = 31556952
It's always been my assumption that someone used 31556952 and everyone else just copied it.