function isLeapYear year
    return (year MOD 4 is 0) AND ((year MOD 400 is 0) OR (year MOD 100 is not 0))
end isLeapYear

command testLeapYear
    set itemDelimiter to comma
    put  "1900,1994,1996,1997,2000" into years
    repeat for each item y in years
        put y && "is" && isLeapYear(y) && return after tyears
    end repeat
    put tyears
end testLeapYear

1900 is false
1994 is false
1996 is true
1997 is false
2000 is true