def format_obsid_as_calendar_date(obsid):
    """Format an obsid as a UTC calendar date, returning the date as a string.

    This makes only sense because our obsids are actually times, expressed as
    integer GPS seconds. The return value will look like `2016-07-24`. This,
    of course, truncates all time-of-day information contained in the JD. Note
    that we convert from the GPS to the UTC timescale, requiring an accurate
    leap-second table.

    """
    from astropy.time import Time
    t = Time(obsid, format='gps')
    return t.utc.iso[:10]
