Because one can use any version of phosim at any date, the validStart/validEnd dates are not useful for identifying the correct camera description--and thus the correct obs_lsstSim version. obs_lsstSim knows which phosim version the camera geometry was created from via description/phosim_version.txt and the description/camera/camera.py generated with that. To ensure that one is using the correct obs_lsstSim for a given set of phosim data, obs_lsstSim should compare its phosim version with the "branch" field in the phosim headers and either except, fail, or issue a warning when they do not match.