The strtotime() function makes it easy for you to work out dates by taking, as it's argument, selected English phrases.

The general format of the strtotime() function is as follows:

strtotime("English Phrase");

This returns a Unix Timestamp.

Example:

echo strtotime("1 week ago");

This outputs:

1327966210

This Unix Timestamp is not very useful to us as you can see. To get a human readable formatted form, combine strtotime() function with the date() function as follows:

echo date("d-m-Y", strtotime("1 week ago"));

This outputs:

30-01-2012

More examples:

echo date("d-m-Y", strtotime("last week"));
echo date("d-m-Y", strtotime("next year"));
echo date("d-m-Y", strtotime("now"));
echo date("d-m-Y", strtotime("tomorrow"));
echo date("d-m-Y", strtotime("next Thursday"));
echo date("d-m-Y", strtotime("1 month ago"));

These will output:

30-01-2012
06-02-2013
06-02-2012
07-02-2012
09-02-2012
06-01-2012

See also:

date()
time()
mktime()
gmdate()
microtime()