The following in theory should work.
Find out the 0 date in Excel, for this example let's assume it's the same 0 date as the datetime data type 1/1/1900.
If the Excel number was 40,000 you would perform the follwing to get the same date in SQL:
SELECT DATEADD(dd,40000,'1 jan 1900')
Obviously you will need to offset the base date to whatever it is in Excel.
Rich