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datetime模块

In [1]: import datetime

In [2]: datetime.datetime.now().strftime('%Y-%m-%d %H:%M')
Out[2]: '2018-08-24 19:03'

In [3]: (datetime.datetime.now()-datetime.timedelta(hours=1)).strftime('%Y-%m-%d %H:%M')
Out[3]: '2018-08-24 18:03'

示例：获取2小时前的时间

import datetime
r = '2小时前'.replace('前', '')
if '小时' in r:
    r = datetime.timedelta(hours=int(r.replace('小时', '')))
elif '分钟' in r:
    r = datetime.timedelta(minutes=int(r.replace('分钟', '')))
now = datetime.datetime.now().strftime('%Y-%m-%d %H:%M:%S')
past = (now - r).strftime('%Y-%m-%d %H:%M:%S')
print(now)
print(past)

2018-08-24 19:11:09
2018-08-24 17:11:09

通用版

import datetime
relative_time = '3天'
units = [('天', 'days'), ('小时', 'hours'), ('分钟', 'minutes'), ('秒', 'seconds')]
for unit in units:
    if unit[0] in relative_time:
        relative = relative_time.replace(unit[0], '')
        relative = eval('datetime.timedelta(%s=%s)' % (unit[1], relative))
        print(relative)
        now = datetime.datetime.now()
        print(now)
        past = (now - relative).strftime('%Y-%m-%d %H:%M:%S')
        print(past)
        break

3 days, 0:00:00
2018-08-24 19:20:42
2018-08-21 19:20:42