Fetching current time from unconnected wifi networks?

802.11 frames don't have any global time information, since such data serves no purpose in the wireless network.

If you have a WiFi hotspot with high uptime, and you can synchronize to UTC time at least once, you could capture beacon frames, which are unencrypted and provide a mandatory timestamp field for synchronization purposes. You could then associate that timestamp with a known UTC value, and later estimate current UTC time using that initial value and the current timestamp. Needless to say, the timestamp is lost every time the hotspot restarts, plus you'll miss some subtle events like leap seconds (that's why I said estimate).

Whether ESP8266 can provide you access to raw beacon frames is another question.