def add_json_datetime_property(i,date='_date',time='_time',format= '{d}T{t}:00',datetime_field='time'):
    """Format datetime field based on given date, time, and format.
    
    Rationale:
        Some applications require a joint date-time variable in a specific format.
        Given an input json feature with properties and the names of fields 
        representing 'date' and 'time', a 'datetime' field is returned.
    
    Args:
        i (geojson feature): This is a geojson feature, containing incorrect
        date (string): Property name of date string
        time (string): Property name of time string
        format (string): A parameterised format for combining these two fields
        datetime_field : A new field to contain the formatted datetime variable
    
    Returns:
        geojson feature: Geojson feature with date time variable
    """
    t = i['properties']['_time']
    d = i['properties']['_date']
    i['properties'][datetime_field] = format.format(d=d,t=t)
    return i