Cannot SUM datetime field in SQL Server migrated from Access
Presumably your new column has values which are a certain datetime relative to 1-Jan-1900, or some other date.

...in which case, you could rename your column, and create a calculated column with your original column name, which is the number of second (or minutes or whatever granularity you want), so that you can SUM that instead.

Example:

SELECT t = DATEADD(SECOND, SUM(DATEDIFF(SECOND, '19000101', myTimeColumn)), '19000101')
FROM dbo.someTable;