Your Guesty listing allows same-day bookings until a cutoff hour, but your HomeRunner website shows no same-day availability at all. The timezone is not the cause.
Why this happens. Guesty publishes two calendars. The one the HomeRunner website integration reads marks every day inside a listing’s advance-notice window as unavailable for the entire day. Guesty’s own booking engine applies the cutoff hour instead.
Check availability first. Before requesting a change, confirm your listings are actually free today. On a busy day, most units are blocked by a reservation or a manual block in Guesty, and your website correctly shows no availability.
What you can do. HomeRunner has a setting on the Guesty connection called Allow Advance Notice Date Selection that lets those days show as bookable on your website. Ask support to enable it on your connection. The calendars re-sync within about half an hour.
Your cutoff rule still applies. If a guest tries to book same-day after the cutoff, Guesty declines the booking at the quote step with the message "the arrival date is too soon to book online".