Due date calculation?

Laura • Our baby Tilly arrived 28th of June, her first breath took ours away!
Hi everyone! I just found out 3 days ago that I'm pregnant, yay! Just wondering what the story is with my due date, my last period was 9th of sept and it was like a 42 day cycle( longest ever, it's usually 37). Glow has predicted that I'm 4 weeks + 2 days preg but another app says I'm 5 weeks+ 6 days?