Erlang Central

# Difference between revisions of "Week of the Year and Day of the Week Month Year"

From ErlangCentral Wiki

## 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.

## Solution

Luckily, this functionality is all supplied in the calendar module.

```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 are no built-in functions for identifying the week of the year, or the day of the year (directly). However, by doing a bit of math we can figure out at least the day of the year:

```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
```

Note: 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.