Math with dates and date ranges is often *fun & enlightening*! As
a testament to the fun of calculating dates (particularly ranges of dates), I
present the following:

Given a date, how would you find the range of dates that describe the week
during which your original date lies? In other words, assume today is *March
7, 2013* (and it is... for now anyway). Can you answer these two questions:

- What was last Sunday's date?
- What will be the date on Saturday?

*NOTE: I'm assuming weeks start on Sunday and end on Saturday. I'm in the
US and that's how people in my area typically define a "week".*

Here's some Python code that calculates this. (also available at https://gist.github.com/bradmontgomery/5110985)

```
from datetime import timedelta
def week_range(date):
"""Find the first/last day of the week for the given day.
Assuming weeks start on Sunday and end on Saturday.
Returns a tuple of ``(start_date, end_date)``.
"""
# isocalendar calculates the year, week of the year, and day of the week.
# dow is Mon = 1, Sat = 6, Sun = 7
year, week, dow = date.isocalendar()
# Find the first day of the week.
if dow == 7:
# Since we want to start with Sunday, let's test for that condition.
start_date = date
else:
# Otherwise, subtract `dow` number days to get the first day
start_date = date - timedelta(dow)
# Now, add 6 for the last day of the week (i.e., count up to Saturday)
end_date = start_date + timedelta(6)
return (start_date, end_date)
```

Now, playing with this in a python shell...

```
>>> from datetime import datetime
>>> d = datetime(2013, 3, 7)
>>> week_range(d)
(datetime.datetime(2013, 3, 3, 0, 0),
datetime.datetime(2013, 3, 9, 0, 0))
```

This is also useful if you've got a Django site, and you want to find the Users that joined during a certain week:

```
>>> from django.contrib.auth.models import User
>>> d = datetime(2013, 3, 7)
>>> week = week_range(d)
>>> User.objects.filter(date_joined__range=week)
[<User ...>, ...]
```

Cool, huh. Check out the docs for `timedelta`

and
`isocalendar`

for more information.