How does your clinic calculate your due date?

(Hey guys, sorry I know this is a sensitive question for those still waiting to find out how their round has gone (and those whose it hasn’t worked this time). Wishing everyone all the best ♥️)

How does your clinic calculate your due date? All the online IVF calendars say I’m 6w3d but today my clinic said I’m 5w5d, as they count back 2 weeks from implantation date. But that doesn’t take into account that it was a 5d blastocyst?? Just annoys me as I figure it should be an exact science, they literally know all the timing.. and now I have to re-live all last weeks fears because I’m almost a week behind what I thought I was 😖