How can I turn the date into Unix format?

As you may already know, Date field for API have to be filled in UnixTimeStamp.

The correct format is to express timestamp in seconds (10 characters - Epoch Timestamp)

📌Please use this useful tool for conversion and actual date check.

📌Keep in mind that 13 characters timestamps are in milliseconds and are not supported.

For more info about how to set it, take a look here.