How to converte EPOCH time into time and date on Arduino?
Epoch are seconds from 1970-01-01. Get it from JSON as unsigned long. They are a number in the returned JSON (no ") 

from https://openweathermap.org/ doc:

"sys":{"country":"JP","sunrise":1369769524,"sunset":1369821049},



to convert it to string with TimeLib.h:

#include &lt;TimeLib.h&gt;
#include &lt;ArduinoJson.h&gt;

void setup() {

  Serial.begin(115200);

  const char* json = "{\"country\":\"JP\",\"sunrise\":1369769524,\"sunset\":1369821049}";

  DynamicJsonBuffer jsonBuffer;
  JsonObject&amp; root = jsonBuffer.parseObject(json);

  unsigned long t = root["sunrise"];

  char buff[32];
  sprintf(buff, "%02d.%02d.%02d %02d:%02d:%02d", day(t), month(t), year(t), hour(t), minute(t), second(t));

  Serial.println(buff);
}

void loop() {
}