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Time-delay cosmography is measurement of cosmological parameters through the use of time delays observed in gravitationally lensed transients, such as lensed supernovae or the activity of lensed quasars. It is of interest as an independent method of measuring the Hubble constant (offering more data in exploring the Hubble tension), and a means of determining past values of the Hubble parameter (offering more data in characterizing the history of dark energy). Gravitational lensing can produce images of the same source (the lensed object) showing at two or more places in the celestial sphere. The lengths of paths over which light travels producing these separate images are not the same, and a lensed object's transient appears to us in these images at different times. The time interval between when the transient shows in each of a pair of images is the time delay of interest. If the mass of the lensing object is known, these delay-times provide data regarding the geometry of the spacetime between the lensed object and the lens.