'The President' is a large-flowered variety of clematis. They usually start flowering from the beginning of June.
These large-flowered varieties produce their blooms from the previous year's growth.
The best time to prune this clematis type is in February or early March. When you can see buds of new growth starting to develop - that's when you can reach for the secateurs. Trim away weak or dead bits and trim the stems lightly.
There are different rules for other types of clematis but it is important that you prune each one correctly.
Do you have a clematis? How does it perform?