NTPClient library - How does timeClient.update() work

Using a ESP8266-12E I've been trying the Network Time Protocol (NTP) but have discover an unexplained issue as follows:

Using say the example sketch that comes with the library 'basic.ino' I find the time displayed updates even when I shut down my WiFi as follows:

void loop() { timeClient.update(); Serial.println(timeClient.getFormattedTime()); delay(1000); }

My expectation is that timeClient.update() should fail but the sketch seems to keep accumulating/updating and displaying time by itself.

I've been unable to find a manual/instructions for library NTPClient.

Assistance / comments welcome.

Presumably, timeClient.update() updates the internal clock and timeClient.getFormattedTime() retrieves a time from the internal clock.

Even if the update fails (having worked at least once) the internal clock will still be running and therefore you can retrieve a time from it.