"The JSON value could not be converted to System.DateTimeOffset. Path: $.startStudy | LineNumber: 1 | BytePositionInLine: 46." code example

Example: The JSON value could not be converted to System.DateTime

your date value in your JSON isn't correct. should be

2019-01-06T17:16:40
Most parsers use ISO 8601

You can use

new Date(stringValue).toISOString()

Then your post will work using the correct date format :)