Depending on the nature of your OLEDB source (is it an RDBMS?), the most efficient way would be to CAST the varchar date/time columns to a single column with a datetime datatype and use a select query with an appropriate WHERE clause to provide the data (and not just the whole table).

An even better way would be to add a proper datetime column to the source data, but I'm assuming that's not allowed? If there's a lot of data in this table and it's growing, any method is going to gradually grind to a halt, unless you are somehow able to get a useful index on it (eg, in SQL Server, on a computed datetime column added to the base table).