Time difference between observations from SDSS and eROSITA. The top panel shows MJD_SDSS–MJD_eROSITA– in years. We highlight the different generations of SDSS as shaded regions, with SDSS I-III observations covering negative values (SDSS spectroscopy preceding eROSITA observations), while SDSS IV–V observations are to the right of the dashed line, indicating spectroscopic observations following the X-ray ones. The bottom panel displays the same time difference divided by (1 + z), showing the time difference in the rest-frame.