day d m y == nameof ((uptoyear y + dayofyear d m y) % 7)

dayofyear d m y == (daysupto m y) + d

daysupto 1 y == 0
daysupto m y == daysin (m-1) y + daysupto (m-1) y

isleap y == true <| y%400 = 0 |> false <| y%100 = 0 |> true <| y%4 = 0 |> false

lengthofyear y == 366 <| isleap y |> 365

uptoyear y == 0 <| y = 1 |> lengthofyear (y-1) + uptoyear (y-1)

daysin 1 y == 31
daysin 2 y == 29 <| isleap y |> 28
daysin 3 y == 31
daysin 4 y == 30
daysin 5 y == 31
daysin 6 y == 30
daysin 7 y == 31
daysin 8 y == 31
daysin 9 y == 30
daysin 10 y == 31
daysin 11 y == 30
daysin 12 y == 31

nameof 0 == "Sunday"
nameof 1 == "Monday"
nameof 2 == "Tuesday"
nameof 3 == "Wednesday"
nameof 4 == "Thursday"
nameof 5 == "Friday"
nameof 6 == "Saturday"