Yes unfortunately the calendar system is pretty unusable. It automatically blocks out dates that a sitter is booked with another THS sit, and shows all other dates as ‘available.’ However, just because a sitter is not booked with another THS sit doesn’t necessarily mean they are ‘available’ to sit, ana it’s tricky to figure out how to manually edit the calendar - most don’t.
I am assuming you are looking at sitter’s calendars to try and send invites? If you’re struggling to find sitters, usually the best way to secure a sitter is to have a really attractive profile that gains applicants, rather than sending invites - unfortunately invites have a rather low success rate (sometimes they work out, but invites rarely match a sitter’s availability or interest).
If you want any feedback & support with you listing, you can link your listing to your forum bio for members to view. Here’s how: How to add a listing or profile link to your FORUM profile
You might also want to check out this blog I inspired for THS about mistakes to avoid when creating a listing.
Hope that helps!