Only time I can water is 9pm after work, is that actually what is giving me mildew every July

Water the soil, not the plant, and the whole timing argument mostly evaporates. I run a soaker hose under the mulch on a cheap tap timer and the leaves never get wet at all, so it genuinely does not matter to me whether it runs at 6am or 10pm. That said, if you are standing there with a hose and a rose fitting after dark, you are wetting foliage that will stay wet for eight hours, which is worth avoiding for reasons other than powdery mildew. The timer is a good buy regardless, mine paid for itself in courgettes the first summer I stopped under watering on hot weeks.

Folklore, mostly, but there is a real thing buried underneath it. What you have on courgettes and cucumbers in July is almost certainly powdery mildew, and powdery mildew is the odd one out among the fungal problems because it does not need a wet leaf to germinate, it does fine in warm dry conditions with humid nights. Downy mildew, blight and most of the leaf spots do need leaf wetness, and for those, evening overhead watering genuinely stacks the odds against you. So the advice is not wrong in general, it is just wrong about the exact disease you have. The fix for your problem is airflow, spacing, and not the hour on the clock.

Mild disagreement with the "midday is fine too" crowd that always turns up in these threads. I watered at lunchtime for a season because I was working from home and it was convenient, and I lost a lot to evaporation, particularly on windy days on open ground with no mulch. Beds needed noticeably more water for the same result. Evening was better than midday for me by a distance, morning slightly better than evening, and the gap between morning and evening was small enough that I would not restructure my day for it.

One more mildew angle since nobody has said it: variety choice does more than anything you can do with a hose. I switched to a courgette and a cucumber that are sold as mildew resistant and the problem went from July to September, which is basically the end of the season anyway. Resistant does not mean immune, but it bought me two months.

Water at night if that is when you are there. Missing a watering does far more damage than the timing ever will. Mulch, space the plants further apart than the packet says, and take out the oldest leaves once mildew starts so air moves.