LibreOffice/Excel handle dates different than you might think...

gistfile1.py

import math
import datetime

secs_per_day = 24 * 60 * 60

def SOMECALCFUNC(datefloat):
  # date comes as float from libreoffice where the integer part is the number
  # of days since 1899-12-30 and the decimal part is the time as fraction of
  # day.
  timefrac, days = math.modf(datefloat)
  dt = datetime.datetime(1899, 12, 30) + datetime.timedelta(days=days, seconds=timefrac * secs_per_day)