You are not able to set the Calculated property to an extension field with type "Date/Type".
"Image/data in this KBA is from SAP internal systems, sample data, or demo systems. Any resemblance to real data is purely coincidental."
SAP Cloud for Customer
The Calculated option is currently not available for extension fields with type "Date/Time".
This properly only can be set for text (Text and Formatted Text) and number (Decimal Number, Amount and Quantity) types. Text fields can be concatenated and numerical fields can be added, subtracted, multiplied, or divided.
This is the standard behavior of the system.
If you need this functionality urgently you may contact your implementation manager to help with such request or approach directly the SAP Cloud Service Center.
You can find more details about the services offered by Cloud Service Center on https://www.sap.com/services/application-development.html . You can contact Cloud Service Center as follows:
1. If you are a partner: please use firstname.lastname@example.org or the Partnerfinder
2. If you are a customer: please use the little blue box “Contact Us” shown on the very right hand side of https://www.sap.com/services/application-development.html, choose “Contact Us”, choose “Services”, choose “SAP Custom Development” and complete the page
Note: The services of the SAP Cloud Service Center will be charged as packaged services based on fixed prices.
An alternative option may be to check the SAP Customer Influencer Site to submit an idea: https://influence.sap.com/SAPCloudforCustomer
Calculated; Date/Time; Time; Date; Extension Field; , KBA , BC-DWB-REP-EXT , Extensibility , How To