ESP32 NTP clock on a TM1637 display

/*
 * ESP32 NTP clock: the time of day from the internet on a TM1637 four-digit
 * display, with the colon blinking once a second.
 *
 *   TM1637   CLK -> GPIO 18 (D18), DIO -> GPIO 19 (D19)
 *            VCC -> 3V3, GND -> GND
 *
 * Libraries: NTPClient (arduino-libraries) and TM1637Display (avishorp),
 * both in the Arduino Library Manager.
 *
 * The board joins WiFi, then NTPClient asks an NTP server for the time over
 * UDP and asks again every 10 minutes. Between answers it counts on from
 * millis(), so the display changes every minute without a request.
 *
 * Set UTC_OFFSET to your time zone in seconds: -5 hours is New York in
 * winter (EST), 3600 is Central Europe in winter, 19800 is India. NTP itself
 * knows nothing of time zones or daylight saving: it answers UTC.
 *
 * In Mokxi the network is SIMULATED and reaches no NTP server, so the answer
 * is the simulation's own clock: 2026-01-01 00:00:00 UTC when you press Run,
 * plus the seconds since. With the offset below the display starts at 19:00,
 * New Year's Eve in New York. On a real ESP32 it shows the real time.
 */
#include <WiFi.h>
#include <WiFiUdp.h>
#include <NTPClient.h>
#include <TM1637Display.h>

const char *SSID = "Mokxi-Lab";
const char *PASSWORD = "breadboard";

const long UTC_OFFSET = -5L * 3600;           // seconds; -5 h is EST
const unsigned long RESYNC_MS = 10UL * 60000; // ask the server every 10 minutes

const int CLK_PIN = 18;
const int DIO_PIN = 19;

WiFiUDP ntpUDP;
NTPClient timeClient(ntpUDP, "pool.ntp.org", UTC_OFFSET, RESYNC_MS);
TM1637Display display(CLK_PIN, DIO_PIN);

// "----" while there is no time yet.
const uint8_t DASHES[] = {SEG_G, SEG_G, SEG_G, SEG_G};

int shownMinute = -1;
bool colon = false;
unsigned long ticked = 0;

void setup()
{
    Serial.begin(115200);
    display.setBrightness(5);
    display.setSegments(DASHES);

    Serial.print("Connecting to ");
    Serial.println(SSID);
    WiFi.begin(SSID, PASSWORD);
    while (WiFi.status() != WL_CONNECTED) {
        delay(500);
        Serial.print(".");
    }
    Serial.println();
    Serial.print("WiFi connected, IP ");
    Serial.println(WiFi.localIP());

    timeClient.begin();
    while (!timeClient.update()) {
        Serial.println("No answer from the NTP server, trying again");
        delay(1000);
    }
    Serial.print("Time set: ");
    Serial.println(timeClient.getFormattedTime());
}

void loop()
{
    timeClient.update(); // asks the server only when RESYNC_MS has passed

    // Blink the colon: on for half a second, off for half a second.
    unsigned long now = millis();
    if (now - ticked >= 500) {
        ticked = now;
        colon = !colon;
        int hhmm = timeClient.getHours() * 100 + timeClient.getMinutes();
        display.showNumberDecEx(hhmm, colon ? 0b01000000 : 0, true);

        if (timeClient.getMinutes() != shownMinute) {
            shownMinute = timeClient.getMinutes();
            Serial.print("The time is ");
            Serial.println(timeClient.getFormattedTime());
        }
    }
    delay(10);
}