Things ticked by in the end for me this week, today I took four calls all wanting to book in in less than an hour under my advertised finishing time! I finish at 6pm, why phone at 5.15 to try and book in for an hour long massage... Grrr why do they do that! 
They're looking for a reason to call without having to actually turn up. I observe this weird phenomenon every time I put on my profile something like 'Sunday last day in London', and I get a flurry of calls asking to book me for Monday. It's a weird form of time wasting
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And other guys just don't read, or it's a strange form of boundary pushing, trying to make you change your plans for them. If you do, they cancel, in a form of controlling and manipulative behaviour. Very odd, but observed over years....
Also, it's the time slot. Lots of guys get off work between 5 and 6, so if you shut the shop at 6, you're not able to see them just when they're coming free in that gap between leaving work at 5 and having to be home for dinner at 7. For lots of men, it's their one opportunity of the week to get laid.