You can put current date and time in download url or/and filename. Please see examples below.

For example time now: 31 October 2012, 16:59:02

[yyyy] = 2012

[yy] = 12

[MM] = 10

[dd] = 31

[HH] = 16

[mm] = 59

[ss] = 01

Source url: http://example.com/date=[dd]&year=[yyyy]&shortyear=[yy]&month=[MM]&time=[HH]:[mm]:[ss]

Parsed url: http://example.com/date=31&year=2012&shortyear=12&month=10&time=16:59:02