The Auger Engineering Radio Array (AERA), part of the Pierre Auger Observatory, is a facility
designed to detect radio emissions from extensive air showers at high energies. Consisting of 153
autonomous radio-detector stations spread over 17 km^2, it detects radio waves in the frequency
range of 30 to 80 MHz. Accurate characterization of the detector response is essential for correct
data interpretation, previously achieved through laboratory measurements of the analog chain
and measurements of the antenna’s directional response. In this study, we perform an absolute
calibration using the continuously monitored sidereal modulation of the diffuse Galactic radio
emission. Calibration is done by comparing the average spectra recorded by the stations with
seven different models of the full radio sky propagated through the system response, including
antennas, filters, and amplifiers. The Galactic calibration is in good agreement with the original
laboratory calibration. In addition, we analyze the time-dependence of the calibration constants
over a period of seven years. No aging effects are observed in AERA stations over a timescale
of a decade, which shows that radio detectors could help monitor possible aging effects of other
detector systems during long-term operations and highlight their importance in determining an
absolute cosmic-ray energy scale.