This works GREAT: (your code)

SELECT

DATEADD(minute,(128624995457225598*100/POWER(10, 9) - (CONVERT(bigint, DATEDIFF(minute, '1/1/1801', '1/1/1953')) * 60))/60, '1/1/1753') AS date

But as soon as I try to plug in the real column (which had to be imported as a varchar) I get this error:

Arithmetic overflow error converting expression to data type bigint.

SELECT

DATEADD(minute,(convert(bigint,lastlogon)*100/POWER(10, 9) - (CONVERT(bigint, DATEDIFF(minute, '1/1/1801', '1/1/1953')) * 60))/60, '1/1/1753') AS date

from dbo.smx_empdirectory