MS SQL Server -- substring DateTime ?

UPDATE [ReportServer].[dbo].[Schedule]
SET
    [StartDate] = CONVERT(char(8), CURRENT_TIMESTAMP, 112) + ' 02:00:00.000',
    [NextRunTime] = CONVERT(char(8), CURRENT_TIMESTAMP, 112) + ' 02:00:00.000'
WHERE
    [Name] LIKE 'EOM - 3:30a%'

Below works, but how can I get the "substring"
to work since I have 50+ updates and want to
use a substring of the original StartDate value without
getting an Argument data type datetime is invalid for
argument 1 of substring function ERROR ?
------------------------------------------------------------------------------
UPDATE [ReportServer].[dbo].[Schedule]
SET StartDate =
         CAST(
                 CAST('2012-03-19' AS varchar) +
                 ' 03:30' AS DATETIME
                          --substring([StartDate],11,13) AS DATETIME
             )
WHERE [Name] LIKE 'EOM - 3:30a%'

Oh, OK, you don't always want 02:00:00.000, you want to retain the original time --
right?.

Btw, the format YYYYMMDD is *universal* in SQL Server and *always* works, but YYYY-MM-DD is not can fail under certain settings.


UPDATE [ReportServer].[dbo].[Schedule]
SET StartDate =
         CAST(
                 '20120319 ' + CONVERT(varchar(20), startDate, 114)
             )
WHERE [Name] LIKE 'EOM - 3:30a%'


SQL will automatically convert the varchar value to datetime if/when needed.