You're looking for the function datediff, however it'll overflow for a value that far back as datediff returns an int and 1900/01/01 is more than MAXINT seconds ago.
This works.
SELECT CAST(datediff(hour,'1900/01/01',GETDATE()) AS BIGINT)*3600 AS SecondsSince1900
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability