Formula date+90 conditional question

I have a list that takes an Open Date and adds 90 days to set a due date:

=[Open Date]+90

However, many of the Open Dates are years old. I've been asked to change it so that dates created before May 31, 2014, should default to August 31, 2014.

I'm thinking it's something like the following, but not sure of the syntax. I've tried these to no avail:

=[Open Date]+90, IF([Open Date]<5/31/2014,"8/31/2014")

=IF([OpenDate]<5/31/2014,"8/31/2014",IF([OpenDate]>6/1/2014,([OpenDate]+90)))

=IF(([OpenDate]+90)<5/31/2014,"8/31/2014",[OpenDate]+90)

Any recommendations greatly appreciated.

Thanks,

Scott