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[edit] Problem

Given a date, you need to find out what week of the year, day of the week, day of the month, or day of the year that the date falls on.

[edit] Solution

These functionality are available using the calendar module, directly or indirectly.

1> {Today,Time} = erlang:universaltime().
{{2004,8,28},{7,59,8}}
2> DayOfWeek = calendar:day_of_the_week(Today).
6
3> {_, Month, DayOfMonth} = Today.
{2004,8,28}
4> DayOfMonth.
28

If you wanted to get the text string representing the day or the month, you can use the httpd_util module (which contains all sorts of useful things that should be moved to the calendar and string modules:

5> DayName = httpd_util:day(DayOfWeek).
"Sat"
6> MonthName = httpd_util:month(Month).
"Aug"

Unfortunately, there is no built-in function for identifying the day of the year (directly). However, by doing a bit of math we can figure it out:

7> Day0 = calendar:date_to_gregorian_days(2004,1,1).
731946
8> TodayDay = calendar:date_to_gregorian_days(2004,Month,DayOfMonth).
732064
9> DayOfYear = TodayDay - Day0.
118

There are different week of the year definitions used by different parts of the world. The calendar module contains the function for determining the week number which conforms to the ISO 8601 standard. (Available in Erlang version R14B02.)

1> calendar:iso_week_number().
{2011,11}
2> calendar:iso_week_number({2006,1,1}).
{2005,52}
3> calendar:iso_week_number({2007,1,1}).
{2007,1}
4> calendar:iso_week_number({2008,12,31}).
{2009,1}

The function returns a 2 tuple with the year and the iso week number. It returns the year because the date can fall on the previous or the next year too!

One very useful thing (beloved of Financial Systems Programmers everywhere) is that Erlang has an end-of-month function:

10> LastDayOfMonth = calendar:last_day_of_the_month(2004, 8).
31
11> calendar:last_day_of_the_month(2004,2).
29
12> calendar:last_day_of_the_month(2005,2).
28

If you are wondering what to use when you are using the Epoch (seconds) representation of the given date, then you should consider transforming your number of seconds to date, and then use this recipe. See recipe TimeEpochToTime to pass from seconds to dates.