1. Create 2 variables vStartDate and vEndDate in setting->Variable overview , give value as your date fields and then create 2 calendar objects and select variable like below, now you are able to select start date and end date,
Firstly, give limits in Min Max values like, Min value = min(dateField) , Max Value = (max(dateField) You need to create 2 different calendar objects, you have created one and given both values. one for start date and one for end Date. And have you given values to variable in Variable overview ?