[Tutor] what's wrong with this ?

[lei yang]

>>>start_time = "2014-7-1"
>>> revlines = commands.getoutput("git log --pretty=format:'%ad:%an'
--date=short --since='%s' --no-merges" %start_time).strip().split('\n')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: unsupported format character 'a' (0x61) at index 26
>>>


if I directly use

revlines = commands.getoutput("git log --pretty=format:'%ad:%an'
--date=short --since='2014-7-1' --no-merges" ).strip().split('\n')

it works well