A diary is simple, you hardly need to use Pages or Word.
Create your own template, I assume you want a current date at the top so either insert a fresh date every day or type it.
Insert a Page Break or Section Break at the end of each day to separate them.
You can create a Table with a formula to insert the current date as well or calculate on the last day plus one.
I will make one appeal to common sense and you can choose to ignore it and probably live to regret it.
Do not put anything you value into a format that is constantly being changed by Apple, can be opened by only one badly written Application in only one OS, maybe, maybe not.
I personally would use a simple Text only App but there are perfectly good free options:
https://www.macupdate.com/app/mac/34880/motodiary
https://www.macupdate.com/app/mac/20923/dreamdiary
https://www.macupdate.com/app/mac/11690/mrdiary
https://www.macupdate.com/app/mac/24635/clover-diary
or a basic neat free Word Processor like Bean
or Evernote or Notes.
Peter