This library only saves seconds will lead to a system error between 0 and 1 second, average 0.5s.

For example, the program synchronizes at 0h0m0 .9s. By the design, it saves time as 0s. After 0.2s, i.e. 0h0m1.1s, I use getFormattedTime(). It should return 0h0m1s, but it will return 0h0m0s. It will not update the time until 0h0m1.9s. As so it will update 2s at 2.9s, 3s at 3.9s. This is a const system error.

The NTP can achieve an accuracy below 10ms. So this about 0.5s error is catastrophic.
issue79 and issue23 relate to this.