I eventually used the following code,
The excel serial data in my scenario always begins with '4'
The code below is fired at the loaded staging table in the database..
thanks for your contributions they are greatly appreciated.
update test_table set Dates = dateadd(d,cast (Dates as int),'1899-12-30') where Dates like '4%'
update test_table set Dates = CASE WHEN ISDATE(Dates) = 1 THEN CONVERT(nvarchar(100),CONVERT(Date,Dates),113)END