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

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()