The expression looks right to me, is there any possibility that the incoming field is not formatted correctly in the source data? The error message seems to indicate a conversion failure, so the string components aren't being reassembled into a valid date. You might consider setting up an exception table, and setting up an error trap to see what the value is. Then you can set the error condition to redirect row instead of fail component. I process a lot of very dirty source data, so all of my packages use a setup like this.

You might also consider throwing a trim call on the inbound varchar prior to substringing it. It can affect performance, but I've had to do it on a few occasions.