conversion failed when converting date and or time from character string

Hi,

I am getting the following error :

conversion failed when converting date and or time from character string

I am using Sql Server 2008.(database designed for sql 2005 later moved to sql server 2008).

Pickup_time and actual_Pickup_time are varchar(5) in database.

What is wrong with this query?


Query:

SELECT COUNT(Trip_ID) AS OntimePickupCount 
FROM MyTABLE
WHERE Start_Dt BETWEEN '01/01/2014' AND '04/30/2014'
AND (DateDiff(minute, CAST (Pickup_Time AS time), CAST (Actual_Pickup AS time )) BETWEEN 0 AND 15
OR DateDiff(minute, CAST (Actual_Pickup AS time), CAST (Pickup_Time AS time) ) BETWEEN 0 AND 15)
AND Actual_Pickup IS NOT NULL AND Actual_Dropoff IS NOT NULL

Thanks