Hi @Bhuvaneshwari Balasubramaniam ,
You need to check whether hiredate is filled in User Profile properties of SPO Admin center. Please do so by navigating through More Features> User Profiles > Manage User Profile.
Also note that hireDate property is specific to SharePoint Online:
For EmployeeHireDate: We can store the employee data like birthday and employeehiredate in azure AD through couple of ways.
One way is to use azure ad connect (if you have on premise active directory) and the other way is using schema extensions. You need to use Microsoft Graph API to work with azure ad schema extension, there is no direct option to extend the schema from azure portal.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have further questions about this answer, please click "Comment".