The global settings should be defined before using Calendar.
Navigate to Jira Administration → Apps → Teamlead Calendar
Review the following settings:
- Default Start Date Custom Field – the Date Time Picker field should be selected.
- Default End Date Custom Field – the Date Time Picker field should be selected.
Each issue should have a start date&time and end date&time to be displayed within a calendar view. This global setting allows you to select the pair of fields with Date Time Picker type as the default options that will be suggested to a user during a new calendar creation. This global setting can be changed for a particular calendar if required.
By default, the Start & Finish custom fields are selected. These fields are provided by the app and can be managed as usually within the list of custom fields of your Jira. Also, you can choose other Date Time Picker fields for the calendar view.
It's not necessary to add the fields on screens. They will work if filled out with values, so you can put them on transition screens or define them by using some automation.