Some basic functions for a Gregorian calendar

gregorian.erl

-module(gregorian).
-export([
    days/0,
    months/0, 
    dayTuples/0,
    monthTuples/0,
    dayOfWeek/1,
    monthOfYear/1,
    isLeap/1,
    daysOfMonth/2,
    daysOfYear/1,
    prev/2,
    next/2,
    prevMonth/1,
    nextMonth/1,
    toDays/1,
    toDays/2,
    toDays/3,
    daysBetween/2,
    daysSince/3
]).

days() -> [ mon, tue, wed, thu, fri, sat, sun ].
months() -> [ jan, feb, mar, apr, may, jun, jul, aug, sep, oct, nov, dec ].

dayTuples() -> lists:zip(lists:seq(1, 7), days()).
monthTuples() -> lists:zip(lists:seq(1, 12), months()).

dayOfWeek(N) when is_integer(N) -> lists:keyfind(N, 1, dayTuples());
dayOfWeek(N) when is_atom(N) -> lists:keyfind(N, 2, dayTuples()).

monthOfYear(N) when is_integer(N) -> lists:keyfind(N, 1, monthTuples());
monthOfYear(N) when is_atom(N) -> lists:keyfind(N, 2, monthTuples()).

isLeap(Y) ->
    if Y rem 400 =:= 0 -> true
     ; Y rem 100 =:= 0 -> false
     ; Y rem 4 =:= 0 -> true
     ; true -> false
    end.

daysOfMonth(M, Y) when M =:= feb ->
    case isLeap(Y) of
        true -> 29;
        false -> 28
    end;
daysOfMonth(M, _) when true ->
    case lists:member(M, [ apr, jun, sep, nov ]) of
        true -> 30;
        false -> 31
    end.

daysOfYear(Y) -> lists:sum([ daysOfMonth(M, Y) || M <- months() ]).

prev(E, [ H | T ]) ->
    case E =:= H of
        true -> lists:last(T);
        false -> lists:last(lists:takewhile(fun(X) -> X =/= E end, [ H | T ]))
    end.
next(E, L) ->
    case E =:= lists:last(L) of
        true -> hd(L);
        false -> hd(tl(lists:dropwhile(fun(X) -> X =/= E end, L)))
    end.

prevMonth(M) -> prev(M, months()).
nextMonth(M) -> next(M, months()).

%% Real Gregorian days, not ISO standard Year 0 BS!
toDays(Y, M, D) -> D - 1 + toDays(Y, M).
toDays(Y, M) when M =:= jan -> toDays(Y);
toDays(Y, M) when true ->
    P = prevMonth(M),
    daysOfMonth(P, Y) + toDays(Y, P).
toDays(Y) when Y =< 1 -> 0;
toDays(Y) when true -> daysOfYear(Y) + toDays(Y - 1).

daysBetween({Y1, M1, D1}, {Y2, M2, D2}) -> toDays(Y2, M2, D2) - toDays(Y1, M1, D1).

daysSince(Y, M, D) ->
    {{CY, CMN, CD}, _} = calendar:now_to_local_time(erlang:timestamp()),
    CM = element(2, monthOfYear(CMN)),
    daysBetween({Y, M, D}, {CY, CM, CD}).