Just want to confirm my understanding regarding TDAC

I travel from the UK to Thailand on 23rd December, i tried to fill out the form on the official website but my dates were greyed out and unselectable, a quick search showed i can only fill this out within the 72hrs prior to my visit, i know it’s all there in black and white but this correct?

Also the same appears to be true for MDAC? As i’ll be then travelling to Malaysia on 26th, i haven’t confirmed the method of transport yet for this leg

Anything else to consider?

Thanks in advance