from typing import Union


def _get_reception_time_from_scene_dir_second(scene_dir_second: str) -> Union[str, None]:
    """If there is time datum inside `scene_dir_second` string, then return it.
    Otherwise, return None."""

    # second part of scene dir, it can be: `13_53_00`,
    # `13_53_00_ETC2`, `14_35_23_CB11_SIR18`, etc.
    second_part = scene_dir_second.split('_')

    # I need at least three elements to create the time datum
    if len(second_part) < 3:
        return None

    # get the first three elements from the list
    second_part = second_part[0:3]

    # iterate over the list to check if all elements are numbers.
    # if there is an element that is not a number, then return None
    for part in second_part:
        if not part.isdigit():
            return None

    # if all parts are numbers, then join them to create time datum
    return ':'.join(second_part)