You are searching a text field for a date range - if the notes fields for many customers get large....
It would be much easier if rather than a "Notes" field in the Client Table, you used a Notes Table.
Each Note would have an auto-entered creation date field.
Concantenating (putting together) data is always easier than Parsing (extracting) data.
Following up on David's suggestion, if you are using FMP 11 you can look at the starter solution file "Contact Management" it shows how to do the separate Notes table and even a script to create new record with timestamps and account name suitable for future finds and/or you can put the notes in a portal filtered by contact name and the two dates, etc.
For reasons I will never understand this was removed from the fmp12 versions, <mini rant erased> but if you can get a copy of it you can convert it to a fmp12 file.
Thank you #Bumper & David. I shall try out the contacts fp7 file idea first as it 'seems' easier!