A nice day out

I was trying to use the following formula to add a number of weeks to a
date, where $b is the number of weeks.
date('Y-m-d', strtotime($Expiry_Date)+$b*86400*7)
However sometimes it added one day less than it should have done.
$Expiry_Date was always a Saturday and sometimes the answer was a Saturday
and sometimes the previous Friday.

Has anyone any idea why?

TIA
Regards
Paul


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