Check and convert POSIXct objects to strings
assign_departure(datetime)A list with the date and time of the trip departure as
characters.
An object of POSIXct class.
Other assigning functions:
assign_decay_function(),
assign_drop_geometry(),
assign_max_street_time(),
assign_max_trip_duration(),
assign_mode(),
assign_opportunities(),
assign_points_input(),
assign_shortest_path()