In com.apple.airport.preferences.plist (I googled):
<key>ChannelHistory</key>
<array>
<dict>
<key>Channel</key>
<integer>3</integer>
<key>Timestamp</key>
<date>2016-12-22T13:08:30Z</date>
</dict>
</array>
Flag: 2016-12-22-22:08:30