The Once Off trigger will trigger a build at an exact point in time. This trigger will only work once at the specified time and datadate. If you want to run this trigger again you need to delete the trigger and re-create it.
The icon next to the Date & Time field is the data picker helper. Clicking it will display the following calendar.
As you click on the dates and times, the input field will be populated with the values you pickdate time field's value can be changed by typing in the value for the field or using shortcuts. The shortcuts for this field and all other date/time fields in Continua are explained in the Time Triggers section.