def get_expstart(rootname):
    """Return the exposure start time (``expstart``) for the given
    group of files.

    The ``expstart`` is gathered from a query to the
    ``astroquery.mast`` service.

    Parameters
    ----------
    rootname : str
        The rootname of the observation of interest (e.g.
        ``jw86700006001_02101_00006_guider1``).

    Returns
    -------
    expstart : float
        The exposure start time of the observation (in MJD).
    """
    return 5000.0
