It appears that the Excel “serial date” format is actually the number of days since 1900-01-00, with a fractional component that’s a fraction of a day, based on http://www.cpearson.com/excel/datetime.htm. (I guess that date should actually be considered 1899-12-31, since there’s no such thing as a 0th day of a month)
So, it seems like it should be:
def excel_date(date1):
temp = dt.datetime(1899, 12, 30) # Note, not 31st Dec but 30th!
delta = date1 - temp
return float(delta.days) + (float(delta.seconds) / 86400)