Hi everyone

I was hoping to book flights using Amex voucher/Avios for 4th May 2022, which I calculated as 355 days from today's date. However, the calendar on BA.com only shows availability up to 3rd May, which suggests I need to try tomorrow. Have I calculated wrongly or is there something I'm missing?
I want to clarify so I don't mess up the return date, as I know I'll need to be accurate in terms of when I call to add it on.

Thanks