Filemaker finds a span of dates by 1/1/2000...1/31/2000 will find Jan 1 thru Jan 31, 2000
If you have a field StartDate and another field EndDate and another field (calculation - StartDate & "..." & EndDate) a Find based on this calculation field will return January 2000 records.
Can you have text boxes in FM which are not linked to a table/field?
If you can, then can you then perform a find from the values in those text boxes?
In FileMaker, we use field with global storage field options for this purpose. Often, it's useful to define a separate table just for these global fields.
Here's a sample script that uses the dates in two global date fields: gDate1 and gDate2 to perform a date range query:
Enter Find Mode // clear the pause checkbox
Set Field [YourTable::YourDateField ; YourTable::gDate1 & "..." & YourTable::gDate2]
Set Error capture [on] // suppress the error dialog if no records are found
Keep in mind that the set field step won't work unless gDate1 and gDate2 have been defined with the Global Storage field option.