Is there a way to show the date of an entry with an additional set time. ie 90 days?
Not natively within EE's standard tags.
Entry dates are stored as timestamps though so you could use a maths plugin to add on 7776000000 (90 days in milliseconds) then use date formatting to turn that into a nice date.
But that's getting a bit "tag soupy" so you might be better off with a custom plugin that would return the modified, formatted date for you.
Or even more simply, use a plugin that's already out there such as DT