attributes: The feature attributes. It is a JSON object that contains a dictionary of name-value pairs. The names are the feature field names. The values are the field values and they can be any of the standard JSON types - string, number and boolean. Note that date values are encoded as numbers. The number represents the number of milliseconds since epoch (January 1, 1970) in UTC.

is what the esri rest endpoint help says

attributes: The feature attributes. It is a JSON object that contains a dictionary of name-value pairs. The names are the feature field names. The values are the field values and they can be any of the standard JSON types - string, number and boolean. Note that date values are encoded as numbers. The number represents the number of milliseconds since epoch (January 1, 1970) in UTC.

That is what the Esri REST endpoint help says.