Bulk Copy from CosmosDB: DateTimeDiff is not a recognized built-in function name

Hi,
I am posting here and not in the cosmosDB forum, because I can use this query in any tool that allows to query cosmosDB (e.g. directly in the azure portal).

I need the function DateTimeDiff in a query of a "copy data" source. But using functions seems like not to be possible. And I am wondering why? 

I couldn't find a spec that says that cosmos db functions are not available. Probably an old cosmosDB api version is being used in Azure Data Factory? 

Best regards
Holger