Here is
one such site. However a quick and easy method, assuming your departure and return dates are before July 31, is to take the same date in April and add one to it. Thus, for example, if your departure date is July 3, go back three months to April 3 and
add one, coming up with April 4 as the 90th day. For a date in early-mid August, go back to the same date in May and
add two.