FileMaker stored find criteria cannot evaluate a calculated expression. But a script can enter find mode and use set field steps to enter criteria into specified fields. The calculated result parameter of set field can evaluate to produce a calculated result to use as criteria:
Set Field [YourTable::LastSpeech ; ">" & Get ( CurrentDate) - 90) ]
When Setting up Set Field, there are two Specify buttons that must be clicked. To get Set Field [Table::Field ; Expression], add set field to your script and click the first button (specify target field). Select Table::Field from the list of fields. Do not click the specify button next to the repetition box. Click OK to close this dialog box. Now click the lower specify button (calculated result) and create the expression to the right of the semicolon (;). Do not try to type in the semicolon.
Good grief! I just had a brain lapse...I knew that. Thank you for reminding me. When you don't do this all the time, it seems I have to relearn some every time.