As you probably already know, Microsoft has introduced some time ago the Employee hire date attribute in Microsoft Entra ID (formerly known as Azure AD).
Well, good news as you can now use this attribute in dynamic group.
The Employee hire date (employeeHireDate) is a string type.
You can use either Greater Than or Equal or Less Than or Equal operators.
When using the Entra ID portal to create the dynamic group, you can use the ‘simple’ date format, such as 2023-09-01.
When using PowerShell to create the dynamic group, you have to use the sortable format, such as 2023-09-01T00:00:00Z.
NOTE if you use Azure AD Connect to synchronize your Active Directory you will need to select a custom AD attribute and update your synchronization rules to map this attribute with the employeeHireDate attribute (both inbound and outbound)