You could make a UDF that contains the logic and then use it inline. Primary contents would be

DECLARE

@jDate INT,

@jBase INT

SET @jBase = 998799780 -- 1900-01-01

SET @jDate = 1056915413 -- this is the parameter you set to your incoming 'number'

SELECT DATEADD(n, (@jDate - @jBase), '1900-01-01 00:00')

HTH,