how to only use information that is known at a given time python code example

Example 1: datetime decreasing date python

from datetime import datetime, timedelta

d = datetime.today() - timedelta(days=days_to_subtract)

Example 2: python run things at certain datetimes

import datetime as DT
import time

while True:
    now = DT.datetime.now()
    target = DT.datetime.combine(DT.date.today(), DT.time(hour=8))
    if target < now:
        target += DT.timedelta(days=1)

    time.sleep((target-now).total_seconds())
    # do something