With new improvements in sdk both for esp32 and esp8266, this library has become less usefull. Using internal functions of IDF or NonOS (esp8266) have several advantages and it is mandatory for certain scenarios, like using HTTPS with certificate check.
For that reason this library will be deprecated. I will keep the repository opened for archive reasons, but I will not make any big development on it.
I'll copy the recommended NTP initialisation code for both ESP8266 and ESP32 using Arduino. It is easy and convenient.
Arduino MKR1000 users, if any, can keep using it.
(23/nov/2020) - Due to a current project I needed having NTP synchronization with millisecond accuracy. It is not currently implemented in any NTP client library or internal Espressif SDK. So, I've started a new library that focuses on high accuracy sync. It only supports ESP8266 and ESP32.
You can find it on GitHub repository https://github.com/gmag11/ESPNtpClient
It relies in internal Posix Time implementation so it is integrated on SDK time calculation. This means that it can be used with TLS mechanism to check certificates validity.
There are some NTP client examples around. You can see some examples, like this.
In a device like ESP8266 or any Internet connected Arduino having NTP synchronization is well convenient. Using that example you can add NTP client to your projects but I was looking for a quicker way to add it, using something like NTPClient class.
So I decided to do my own NTP client library to sync ESP8266 time via WiFi using Arduino IDE. It can also be used on any ethernet or WiFi connected Arduino, although as I do not have any WiFi enabled Arduino code is not tested. Testers are welcome.
This library support both Arduino MKR1000, ESP8266 and ESP32.
This is a NTP library to be able to get time from NTP server with my connected microcontrollers. Support for regular Arduino with ethernet shield, ESP8266, ESP32 and Arduino MKR1000 is available. Please test it and inform via GitHub.
Using the library is fairly easy. A NTP singleton object is created inside library. You may use default values by using NTP.begin() without parameters. After that, synchronization is done regularly without user intervention. Some parameters can be adjusted: server, sync frequency, time offset.
You don't need anything more. Time update is managed inside library so, after NTP.begin() no more calls to library are needed.
Update frequency is higher (every 15 seconds as default) until 1st successful sync is achieved. Since then, your own (or default 1800 seconds) adjusted period applies. There is a way to adjust both short and long sync period if needed.
In current version source code is the same for all platforms. There has been some interface changes during last update. Although I've tried to keep backwards compatibility you may find some discrepancies. Let me know so that I can correct it.
This library includes an uptime log too. It counts number of seconds since sketch is started. It can be checked calling NTP.getUptime() or NTP.getUptimeString() for a human readable string.
Every time that local time is adjusted a ntpEvent is thrown. You can attach a function to it using NTP.onNTPSyncEvent(). Indeed, this event is thrown just before time is sent to [Time] Library. Because of that, you should try not to make time consuming tasks inside event handler. Although it is taken into account inside library, it would add some offset to calculated time. My recommendation is to use a flag and process it inside loop()function.
Called function format must be like void eventHandler(NTPSyncEvent_t event).
ESP8266 example uses a simple function to turn a flag on, so actual event handling code is run inside main loop.
Please check examples folder into repository source code.
Don't expect atomic-clock-like precision. This library does not take network delay into account neither uses all NTP mechanisms available to improve accuracy. It is in the range of 1 to 2 seconds. Enough for most projects.
I have the plan to add full network delay compensation. Due to limited Time Library precision of 1 second, it probably will not affect overall accuracy.
This library makes use of Time library. You need to add it to use NTPClientLib