acachewilltypicallyusethevalueofthecachedmessage'sLast-ModifiedfieldtogeneratethefieldvalueofIf-Modified-Since.Thisbehaviorismostinteroperableforcaseswhereclocksarepoorlysynchronizedorwhentheserverhaschosentoonlyhonorexacttimestampmatches(duetoaproblemwithLast-Modifieddatesthatappeartogo"backintime"whentheoriginserver'sclockiscorrectedorarepresentationisrestoredfromanarchivedbackup