Fix rough bug in dates:diff

Month difference calced as abs(year1 - year2) * 12 + abs(month1 - month2), what is wrong (same for querters diff)
Example: diff(Jan 2012, Feb 2013) = 13 - correct, diff(Feb 2012, Jan 2013) = 13 - incorrect, correct one is 11
I'll add some tests for such cases a bit later