Search Object with data range

Hi all,

i want to have an api in my service that return a list of objects created in a time range.

I have create this method in the cloud code js (the model is 'Objects'):

----

/**

*

* @param {DateTime} startDate

* @param {DateTime} endDate

* @returns {Promise.<void>}

*/

getObjectsByRange(startDate, endDate) {

var whereClause = "created at or after "+startDate + " and created at or before "+endDate;

var queryBuilder = Backendless.DataQueryBuilder.create().setWhereClause( whereClause );

var result = Backendless.Data.of( Objects ).find( queryBuilder );

return result;

}

------

but when i tested with "Api Service", return this error "400 - Invalid where clause (1017)"

What am i doing wrong?

Thank

GF