If you have missed cutting back your boxwood trees or hedges in July, they can still be cut in September or later. You only need to consider that when cutting this late, the cutting wounds will not heal completely. So if you do not find the time to cut back your boxwood in summer we recommend to leave it till at least mid-March of the next year - before the growth phase starts again.
Other time variants for cutting boxwood and hedges
For cutting boxwood trees or hedges, timewise there are two different variants :
Cut back at the end of May, beginning of June. This way you can use the second growth phase which usually occurs around end of June for a more dense growth of the hedge or boxwood tree. You may then cut back again till mid or end of July.
Cut back at mid or end of July. This way you only need to cut back your hedge or boxwood plants once a year. However, if you want to encourage your boxwood or hedges to grow more dense, you should cut back twice a year using cutting variant 1.