Hi, @MikeZheng-3371 Thanks for the question and for using the MS Q&A platform.
You can use $currentDate here you can find the documentation. Cosmos DB API for MongoDB will follow the same format for date and timestamp fields as MongoDB, and the time will be in UTC, as there is no timezone concept.
Click here for reference doc
Please let me know if you need any additional information.
Regards
Geetha