How to convert a python datetime.datetime to excel serial date number

Question

I need to convert dates into Excel serial numbers for a data munging script I am writing. By playing with dates in my OpenOffice Calc workbook, I was able to deduce that '1-Jan 1899 00:00:00' maps to the number zero.

I wrote the following function to convert from a python datetime object into an Excel serial number:

def excel_date(date1):
    temp=dt.datetime.strptime('18990101', '%Y%m%d')
    delta=date1-temp
    total_seconds = delta.days * 86400 + delta.seconds
    return total_seconds

However, when I try some sample dates, the numbers are different from those I get when I format the date as a number in Excel (well OpenOffice Calc). For example, testing '2009-03-20' gives 3478032000 in Python, whilst excel renders the serial number as 39892.

What is wrong with the formula above?

*Note: I am using Python 2.6.3, so do not have access to datetime.total_seconds()

Answer 1

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)

Answer 2

While this is not exactly relevant to the excel serial date format, this was the top hit for exporting python date time to Excel. What I have found particularly useful and simple is to just export using strftime.

import datetime
current_datetime = datetime.datetime.now()
current_datetime.strftime('%x %X')

This will output in the following format '06/25/14 09:59:29' which is accepted by Excel as a valid date/time and allows for sorting in Excel.

Answer 3

if the problem is that we want DATEVALUE() excel serial number for dates, the toordinal() function can be used. Python serial numbers start from Jan1 of year 1 whereas excel starts from 1 Jan 1900 so apply an offset. Also see excel 1900 leap year bug (https://support.microsoft.com/en-us/help/214326/excel-incorrectly-assumes-that-the-year-1900-is-a-leap-year)

def convert_date_to_excel_ordinal(day, month, year) :

    offset = 693594
    current = date(year,month,day)
    n = current.toordinal()
    return (n - offset)

Answer 4

With the 3rd party xlrd.xldate module, you can supply a tuple structured as (year, month, day, hour, minute, second) and, if necessary, calculate a day fraction from any microseconds component:

from datetime import datetime
from xlrd import xldate
from operator import attrgetter

def excel_date(input_date):
    components = ('year', 'month', 'day', 'hour', 'minute', 'second')
    frac = input_date.microsecond / (86400 * 10**6)  # divide by microseconds in one day
    return xldate.xldate_from_datetime_tuple(attrgetter(*components)(input_date), 0) + frac

res = excel_date(datetime(1900, 3, 1, 12, 0, 0, 5*10**5))
# 61.50000578703704