It seems that there is a problem with correct date time validation on server side resulting in getting code 400 response. It prevents to acquire set of Time Entries started within selected time range.
Here is an example request URL returning Invalid start date error, where I can't find any incorrectness according to your docs:
Client error: GET https://www.toggl.com/api/v8/time_entries?start_date=2018-08-01T00%3A00%3A00%2B0200&end_date=2018-08-12T23%3A59%3A59%2B0200 resulted in a 400 Bad Request response: Invalid start date
OK. I've found source of the problem. There is an ISO 8601 incompatible datetime pattern defined as DateTime::ISO8601 constant (described here). DateTime::ATOM should be used instead.
It seems that there is a problem with correct date time validation on server side resulting in getting code 400 response. It prevents to acquire set of Time Entries started within selected time range.
Here is an example request URL returning
Invalid start date
error, where I can't find any incorrectness according to your docs:Client error: GET https://www.toggl.com/api/v8/time_entries?start_date=2018-08-01T00%3A00%3A00%2B0200&end_date=2018-08-12T23%3A59%3A59%2B0200 resulted in a 400 Bad Request response: Invalid start date