Is there any good idea to ceil or floor datetime in python3?

You can use the datetime.combine(date, time) to create new datetime object using given datetime.date and datetime.time.

Floor

from datetime import datetime
floored = datetime.combine(roughtime.date(), datetime.min.time())

Ceil

from datetime import datetime, timedelta

if roughtime.time() == datetime.min.time():
    ceiled = roughtime
else:
    ceiled = datetime.combine(roughtime.date()+timedelta(days=1), datetime.min.time())

• Ceiling needs if..else statement since if you would ceil 2020-07-24 00:00:00 one would not expect to get 2020-07-25 00:00:00
• ceiled = roughtime is okay (no need to make a copy), since datetime objects are immutable. (source)

You can use replace function to replace the time part to make it floor and add one day to make it ceil

In [1]: import datetime

In [2]: def ceil(datetimeObj):
   ...:     return floor(datetimeObj) + datetime.timedelta(days=1)
   ...:

In [3]: def floor(datetimeObj):
   ...:     return datetimeObj.replace(second=0, hour=0, minute=0)
   ...:

In [4]: roughtime = datetime.datetime(2018,1,1,5,2,1)

In [5]: ceil(roughtime)
Out[5]: datetime.datetime(2018, 1, 2, 0, 0)

In [6]: floor(roughtime)
Out[6]: datetime.datetime(2018, 1, 1, 0, 0)