>How do you determine a number of days in a February, given a year?

In addition to Borislav's answer, there's also a formula for it:

Every year evenly divisible by 4 (MOD(lnYear, 4) = 0) has a leap year,
Except for every century not evenly divisible by 400 (MOD(lnYear, 400) != 0).

1600 = leap year
1700 = no leap year
1800 = no leap year
1900 = no leap year
2000 = leap year
2100 = no leap year
2200 = no leap year
2300 = no leap year
2400 = leap year

We were unique in an almost 800 year time span, in that we saw the century with a leap year.