Timezone / Location

Device can request advanced timezone / location information:

Timezone:        Europe/Kiev
ISO-8601 time:   2020-10-06T10:16:26+03:00
Unix time (UTC): 1601968586694
TZ offset:       120 minutes
DST offset:      60 minutes
POSIX TZ rule:   EET-2EEST,M3.5.0/3,M10.5.0/4
Next offset changes: 120min. on 1603587600, 180min. on 1616893200
Location:        41.941692,-88.390674

Example:

BLYNK_CONNECTED() {
    // Send requests for different internal data
    // Request what is actually needed for your use-case
    Blynk.sendInternal("utc", "tz_name");   // Name of timezone
    Blynk.sendInternal("utc", "iso");       // ISO-8601 formatted time
    Blynk.sendInternal("utc", "time");      // Unix timestamp (with msecs)
    Blynk.sendInternal("utc", "tz");        // Timezone and DST offsets
    Blynk.sendInternal("utc", "tz_rule");   // POSIX TZ rule
    Blynk.sendInternal("utc", "dst_next");  // Up to 2 next time offset changes (due to DST)
}

// Receive UTC data
BLYNK_WRITE(InternalPinUTC) {
    String cmd = param[0].asStr();
    if (cmd == "time") {
        uint64_t utc_time = param[1].asLongLong();
        Serial.print("Unix time (UTC): "); Serial.println(utc_time);
        
    } else if (cmd == "iso") {
        String iso_time = param[1].asStr();
        Serial.print("ISO-8601 time:   "); Serial.println(iso_time);
        
    } else if (cmd == "tz") {
        long tz_offset = param[1].asInt();
        long dst_offset = param[2].asInt();
        Serial.print("TZ offset:       "); Serial.print(tz_offset);  Serial.println(" minutes");
        Serial.print("DST offset:      "); Serial.print(dst_offset); Serial.println(" minutes");
    
    } else if (cmd == "tz_name") {
        String tz_name = param[1].asStr();
        Serial.print("Timezone:        "); Serial.println(tz_name);
    
    } else if (cmd == "tz_rule") {
        String tz_rule = param[1].asStr(); 
        Serial.print("POSIX TZ rule:   "); Serial.println(tz_rule);
   
    } else if (cmd == "dst_next") {
        uint32_t next1_ts  = param[1].asLong();
        int next1_off       = param[2].asInt();

        uint32_t next2_ts  = param[3].asLong();
        int next2_off       = param[4].asInt();
        
        Serial.print("Next offset changes: ");
        Serial.print(next1_off); Serial.print("min. on "); Serial.print(next1_ts);
        Serial.print(", ");
        Serial.print(next2_off); Serial.print("min. on "); Serial.print(next2_ts);
        Serial.println();
    }
}

Tracking local time

It's a common requirement for IoT devices to keep track of local device time. For this, you need to apply the timezone offset, taking into account the Daylight Saving rules. Luckily, there are many ways to implement it, and one of the easiest is to use the ezTime library:

#include <ezTime.h>

Timezone local;
BlynkTimer timer;

BLYNK_CONNECTED() {
    Blynk.sendInternal("utc", "time");      // Unix timestamp (with msecs)
    Blynk.sendInternal("utc", "tz_rule");   // POSIX TZ rule
}

BLYNK_WRITE(InternalPinUTC) {
    String cmd = param[0].asStr();
    if (cmd == "time") {
        const uint64_t utc_time = param[1].asLongLong();
        UTC.setTime(utc_time / 1000, utc_time % 1000);
        Serial.print("Unix time (UTC): "); Serial.println(utc_time);
    } else if (cmd == "tz_rule") {
        String tz_rule = param[1].asStr();
        local.setPosix(tz_rule);
        Serial.print("POSIX TZ rule:   "); Serial.println(tz_rule);
    }
}

void printClock() {
    Serial.println("Time: " + local.dateTime());
}

void setup() {
    // Init Blynk, etc...

    timer.setInterval(10000, printClock);
}

void loop() {
    Blynk.run();
    timer.run();
}

This way, the clock is always running in UTC, but local time is always computed according to the Timezone/DST rule. It works event if device goes offline temporarily.