python求时间差

import datetime
# 求时间差，传入timestart和timestop都是
# 用datetime.datetime.now()的返回值
def timediff(timestart, timestop):
        t  = (timestop-timestart)
        time_day = t.days
        s_time = t.seconds
        ms_time = t.microseconds / 1000000
        usedtime = int(s_time + ms_time)
        time_hour = usedtime / 60 / 60
        time_minute = (usedtime - time_hour * 3600 ) / 60
        time_second =  usedtime - time_hour * 3600 - time_minute * 60
        time_micsecond = (t.microseconds - t.microseconds / 1000000) / 1000

        retstr = "%d天%d小时%d分%d秒%d毫秒"  %(time_day, time_hour, time_minute, time_second, time_micsecond)
        return retstr

备注：python输出普通格式的时间，用str(datetime.datetime.now())[:19]即可，日期则是[:10]。