Oh, yes the penny pincers in accounting will just love the fact that their budget calculations for the next year will entirely consist of statements like "whatever our 1000 code monkeys feel like working even if it exceeds the amount you are willing to pay if they get into 'the zone', best case you wont have to pay them at all because none showed up".
I don't do much dynamic hours like above as I work with/manage others but I do take 40ish days off a year. 2 weeks at Christmas and Easter, 3 in the summer, and 3-5 days off for some of the kids half term holidays.
I prefer just informing the clients when I am unavailable and that is it. Never had an issue since going solo 9 years ago.
There are many downsides to being a contractor, but time off is an upside, if you can afford it.
How is that different from quitting or taking an unpaid sabbatical and hoping the job still exists? I mean, your clients may have found a new contractor in those months.
I bill by the day or by the hour. Client's choice. I even round down to the nearest even hour each day so I don't ever have to have that icky feeling of "did I really bill them right this week?" which would just distract me.
When I work 14 hours in a row, that's what I bill. If I'm in the zone I push it till I fall over. It's worth it for the client. If I'm having an off day, I go home early and bill 4 or 6 hours.
If I'm billing by the day, I just bill by the day. Whether it's 14 hours of working or 4, it evens out and if I'm unsure, I'll bill half a day. The important part is that the work gets done. And if I work a Saturday, you bet it's billed.
Now, why do I say all of this? Because when it comes to time off I vastly prefer my situation. Sick for months? I'm not worrying about whether my PTO qualifies or whatever. I just don't get paid. This has happened recently and when I was healthy again my clients were happy I was able to help again.
This weird sorta dance around time off (sick days, PTO, government holidays, dealing with a manager under pressure for the quarter, etc) makes a bit of sense for the working poor, but I don't understand why so many software developers bother. Just bill what you work and if you want a day, week, or month off take it. I'm sure if more developers asked to work this way large corps would be happy to accommodate them.