There are two ways to make this possible:
Resolution 1
Set up an Event Notification choosing the 'Post Person' event type:
In Portal, Go to People Management, Data Centre, View All
Using the search bar, find HR Event Configuration and select
Choose Post Person from the 'Event Category' dropdown list
Find 'Start Date', three dot ellipsis and 'Edit'
Make your configuration change here
Further guidance is available on setting up an HR Event notification.
Resolution 2
Set the 'Days Before' to something like 300 days for example. This means, anyone you create up to 300 days before the start date, will generate the notification on date entered. You would also have to update 'ignore event days'.
Ignore Event Days: Ignores the event if the date on which the trigger is fired falls after 60 days (default) or more of today. This is where the field is set.
In Portal, Go to People Management, Data Centre, View All
Using the search bar, find HR Event Configuration and select
Select HR Events from the Event Category drop down
Once you find the 'New Starter Notification', click on the 3-dot ellipsis, 'Edit'
Set Ignore Event Days to 300 (or whatever number you have decided on)
Within the individual notification, set 'Generate Notification' to 300
Set 'Days' to 'Before'