Json does not have datetime data type, you need to keep the datetime information in string. This can be a problem for database engines specially if user needs to search by date or sort by date. Cosmos DB team introduced bunch of datetime functions to the Azure Cosmos DB database engine this month. You can read my older post about DateTime in CosmosDB if you like to know how Azure CosmosDB saves the datetime in documents. I will cover the new datetime functions in this post. Here is the list of the functions
Click through for those functions and how you can use them.