Convert an ISO8601 date to the Excel date representation.

to_excel_date.py

from datetime import datetime, date
from dateutil.parser import parse


class ExcelDateException(Exception):
    pass


def to_excel_date(iso_date):
    """Convert a date to the Excel representation of a date
    which is the inclusive range of days since 1900-01-01.

    Time values parsable by `dateutil` will be truncated to a date.

    Excel handles times by dividing the time as seconds by the
    number of seconds in a day, and this fraction is included
    as the fractional-part of the number of days. For instance:

    2017-01-01 == 42736
    2017-01-01 1:35:01 == 42736.0659837963

    The fractional-part calculation is not performed by this function.
    """
    date = parse(iso_date).date()
    delta = date - parse('1900-01-01').date()

    if delta.days == 0:
        excel_date = 1
    elif delta.days > 0:
        # We need to add 2 days to the delta since the datetime
        # subtraction is not inclusive.
        excel_date = delta.days + 2
    else:
        raise ExcelDateException("Excel only converts dates on or after 1900-01-01.")

    return excel_date


def assetRaises(exception, fn, *args):
    failedToRaise = False
    try:
        fn(*args)
    except Exception as e:
        if not type(e) is exception:
            failedToRaise = True
    finally:
        if failedToRaise:
            raise Exception("Expected `%s` to raise %s." % (fn.__name__, exception.__name__))


assert to_excel_date('2017-01-01') == 42736
assert to_excel_date('2017-01-01 1:35') == 42736
assert to_excel_date('1999-12-31') == 36525

assetRaises(ExcelDateException, to_excel_date, '1899-01-01')