Finnish social security number calculator in F#

FinnishSSN.fs

open System

/// Finnish social security number calculator in F#
/// Suomalainen henkilötunnuslaskuri F#:lla
module SSN =
    let getSeparator year =
        if year < 1900 then '+'
        else if year < 2000 then '-'
        else 'A'

    let getNumber female =
        let number = (new System.Random()).Next(2, 899)
        if (number % 2 = 0 && not female) then number+1
        else if female && not (number % 2 = 0) then number+1
        else number

    let getMark date number =
        let marks = [|'0';'1';'2';'3';'4';'5';'6';'7';'8';'9';'A';'B';'C';'D';'E';'F';
                      'H';'J';'K';'L';'M';'N';'P';'R';'S';'T';'U';'V';'W';'X';'Y'|]
        let dateAndNumber = Int32.Parse(sprintf "%s%03i" date number)
        marks.[dateAndNumber%31]

    /// Get finnish social security number
    /// date = birth date of a person
    /// female = true if calculating number for a female
    let getFinnishSSN (date:DateTime) female =
        let separator = getSeparator date.Year
        let number = getNumber female
        let dateString = date.ToString("ddMMyy")
        let mark = getMark dateString number
        sprintf "%s%c%03i%c" dateString separator number mark

// Examples:
SSN.getFinnishSSN (DateTime.Parse("6.6.1992")) false;;
SSN.getFinnishSSN (DateTime.Parse("3.12.2002")) true;;