import datetime and then the magic timedelta stuff:
In [63]: datetime.datetime.now()
Out[63]: datetime.datetime(2010, 12, 27, 14, 39, 19, 700401)
In [64]: datetime.datetime.now() - datetime.timedelta(minutes=15)
Out[64]: datetime.datetime(2010, 12, 27, 14, 24, 21, 684435)
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…