Need trigger to modify current row only

I have a table named CounterData as:

CounterID (int)
CounterValue (int)
CounterDateTime (char 24)

I am trying to write a trigger that will modify the CounterDateTime value for each row as it is inserted into the table. Here is my Trigger as it is working now with the problem/questions to follow:

***
CREATE TRIGGER [Trigger1] ON [dbo].[CounterData]
FOR INSERT, UPDATE
AS
UPDATE CounterData
SET CounterDateTime = CAST(counterdatetime as datetime)
***

Problems:
1.) This trigger method is actually doing the CAST operation on every record in the CounterDateTime column over and over for every time there is an UPDATE or INSERT. I need to make the trigger only manipulate the current record each time ther is an INSERT or UPDATE --- NOT the entire column over again each time.

2.) This trigger does not seem to work when the records are inserted by the application. However if I manually go into the CounterData table and manually update one of the records, the trigger kicks in and corrects the entire CounterDateTime column.

Essentially, I need the trigger to only work with the current record as it is inserted or updated. How can I do this?

You just need a WHERE clause in the update:

UPDATE CounterData
SET CounterDateTime = CAST(counterdatetime as datetime)
WHERE [some condition that identifies the specific row(s) that need updating]

Can I ask why this operation is in a trigger? It seems a little unorthodox to recast data in the same column using a trigger.

Thanks for the reply.

1.) The reason I am recasting data in the same column is that the data is being entered into the table from Windows Perfmon Counter Logs which to my knowledge there is no way to change the format that Perfmon supplies the data in. So, Perfmon is inserting data into the CounterDateTime column in a char(24) format with a bracket character at the end. This format does not let me use datetime comparisons later on in my quieries so I am trying to get the data entered into the table in a casted format that I can use.

2.) As for using the where clause.... Your example says to use Where with [some condition that identifies a specific row]. You are correct, that is exactly what I need to do BUT.... the specific row in this case will always be the specific row that is currently being inserted at any given time. That is my problem - how do I tell sql to only update the CURRENT ROW and not the entire column each time the trigger is run?

Hope this makes sense - I am a newbie.

Oh, I see.

Triggers can operate on the tables "inserted" and "deleted," which are special temporary tables that hold your new or changing rows as an insert or update or delete is processed. Check out BOL under "Using the inserted and deleted Tables." You can use those to isolate only the new rows.