[core] Display modify, expires debug information

src/mbgl/util/time.cpp

@@ -19,5 +19,13 @@ std::string rfc1123(std::time_t time) {
     return buffer;
 }
 
+std::string iso8601(std::time_t time) {
+    std::tm info;
+    gmtime_r(&time, &info);
+    char buffer[30];
+    std::strftime(buffer, sizeof(buffer), "%F %T", &info);
+    return buffer;
+}
+
 }
 }