Simplify the following text for a Grade 10 native English-speaking person.
Original: It is conventionally defined as a period extending from the 16th to the 19th centuries , though climatologists and historians working with local records no longer expect to agree on either the start or end dates of this period , which varied according to local conditions .
Simplified: Climatologists and historians find it difficult to agree on either the start or end dates of this period .