A Formula for the Number of Days in Each Month(cmcenroe.me)

    M   month_offset
    1     1
    2     4
    3     4
    4     0
    5     2
    6     5
    7     0
    8     3
    9     6
   10     1
   11     4
   12     6

   1 4 4

   0 2 5

   0 3 6

   1 4 6

    Thirty days hath September,
    April, June, and November
    And all the rest have thirty-one
    excepting February alone
    And that has twenty-eight days clear
    and twenty-nine in each leap year

    28+((0x3bbeecc>>(m*2))&3)

    30+(m&1^m>7)-m*(m==2)

  // 28 + ((0x3bbeecc>>m>>m)&3);
  b8 cc ee bb 03    mov  eax,0x3bbeecc
  d3 f8             sar  eax,cl
  d3 f8             sar  eax,cl
  83 e0 03          and  eax,0x3
  83 c0 1c          add  eax,0x1c

    days y m = 28 + (m + (m `div` 8)) `mod` 2 + 2 `mod` m + 2 * (1 `div` m) 
               + ((2 `div` m) `mod` 2) * (leap y)
      where leap y = multiple 4 y - multiple 100 y + multiple 400 y
              where multiple int y = ((y + (int - 1)) `mod` int + 1) `div` int

    function f(x) { return 28 + maybeLeap(x) + (...); }