With the development of EUV-lithography, high-accuracy at-wavelength metrology has increasingly gained in importance. Characterization of detectors and sources using synchrotron radiation has been performed by the Physikalisch--Technische Bundesanstalt (PTB) for almost 20 years. At their new laboratory at BESSY II, PTB now has set up instrumentation which is suitable for high-accuracy EUV detector calibration. It uses synchrotron radiation from a bending magnet for detector characterization at a plane grating monochromator beamline. The detector calibration at PTB uses a cryogenic electrical substitution radiometer as the primary detector standard. For the measurement of radiant power of about 1 (mu) W, the systematic uncertainty contributions from the electrical substitution principle of about 0.03 percent relative dominate the measurement uncertainty of the radiometer. Careful adjustment of the temperature control circuit reduced the statistical noise of the measured power to about 0.2 nW. This allows the radiant power to be measured down to 0.1(mu) W with an uncertainty of 0.3 percent or better. This uncertainty is lower than the results achieved elsewhere by more than one order of magnitude. In this paper, the current status of EUV detector calibration at PTB is presented. The high performance of the radiometer, together with the improved stability and spectral purity of the beamline, is illustrated by typical results. In the EUV spectral range, photodiodes can be calibrated with a relative uncertainty of about 0.3 percent. This low uncertainty permits systematic studies of the homogeneity and stability of detectors with unprecedented sensitivity for even minor changes. The responsivity of individual photodiodes has been observed over a period of up to six years. We present a first investigation of the long-term stability of AXUV photodiodes which are widely assumed to be stable in the EUV spectral range. The results are of sufficient accuracy to show that even diodes which are rarely used and carefully stored, degrade. After a period of three years, the degradation becomes ever stronger.