Early August is the last date I would recommend pruning evergreens if you want to be safe from frost damage on the new growth. Boxwood are very tough shrubs however and they defy the rules about pruning so if you want to gamble and give a haircut in September you can probably get away with breaking this rule. Pruning in the fall will not kill your boxwood it may just cause browning on the tips of any new growth if a hard frost moves in to surprise the plants before they are hardened off. If your boxwood look scruffy and you can't stand how they look you have my permission to tidy them up now. Keep growing, Marianne Binetti