Due date?

Cristina • We have 2 girls & expecting baby #3 July 2015
I got my BFP yesterday at 18DPO 
my LMP was 10/15 & I have a 34day cycle.... I check online to caculate my due date but some website tells me July 22 & others July 28 
I'm making an appointment on Monday to confirm but I'm so confuse with this due date
Can anybody help me? 
Thank you

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