Today’s Erlang Thursday covers calendar:date_to_gregorian_days/3.

As we saw in last week’s Erlang Thursday on calendar:day_of_the_week/3 when we were looking at some error messages, we saw that the errors were coming from calendar:date_to_gregorian_days/3.

calendar:day_of_the_week(0, 0, 0).
% ** exception error: no function clause matching calendar:date_to_gregorian_days(0,0,0) (calendar.erl, line 114)
%      in function  calendar:day_of_the_week/3 (calendar.erl, line 151)
calendar:day_of_the_week(1970, 2, 31).
% ** exception error: no true branch found when evaluating an if expression
%      in function  calendar:date_to_gregorian_days/3 (calendar.erl, line 116)
%      in call from calendar:day_of_the_week/3 (calendar.erl, line 151)
calendar:day_of_the_week(1970, 13, 2).
% ** exception error: no function clause matching calendar:last_day_of_the_month1(1970,13) (calendar.erl, line 243)
%      in function  calendar:date_to_gregorian_days/3 (calendar.erl, line 115)
%      in call from calendar:day_of_the_week/3 (calendar.erl, line 151)

I promised you at the end of that post we would take a deeper look at calendar:date_to_gregorian_days/3 next time, so let’s fulfill that promise.

calendar:date_to_gregorian_days/3 takes three arguments, a non-negative integer for the year, an integer between 1 and 12 (inclusive) for the month, and an integer between 1 and 31 (inclusive) for the day of the month, and returns the number of days since 0000-01-01 in the Gregorian calendar.

calendar:date_to_gregorian_days(2015, 4, 16).
% 736069
calendar:date_to_gregorian_days(0, 1, 1).
% 0
calendar:date_to_gregorian_days(1, 1, 1).
% 366
calendar:date_to_gregorian_days(1970, 1, 1).
% 719528
calendar:date_to_gregorian_days(1999, 12, 31).
% 730484

There is also a version calendar:date_to_gregorian_days/1, that takes a tuple of year, month, and day available if your code already has the date in tuple format.

calendar:date_to_gregorian_days({2015, 4, 16}).
% 736069
calendar:date_to_gregorian_days({0, 1, 1}).
% 0
calendar:date_to_gregorian_days({1, 1, 1}).
% 366

And if we pass something invalid to calendar:date_to_gregorian_days/1, we see that it is calling calendar:date_to_gregorian_days/3. So it is just a nice helper function that does the pattern match destructing for us.

calendar:date_to_gregorian_days({1, 1, 0}).
** exception error: no function clause matching calendar:date_to_gregorian_days(1,1,0) (calendar.erl, line 114)

–Proctor