[core] Display modify, expires debug information

include/mbgl/util/time.hpp

@@ -1,6 +1,8 @@
 #ifndef MBGL_UTIL_TIME
 #define MBGL_UTIL_TIME
 
+#include <mbgl/util/chrono.hpp>
+
 #include <string>
 #include <cstdint>
 #include <ctime>
@@ -12,6 +14,9 @@ namespace util {
 // Returns the RFC1123 formatted date. E.g. "Tue, 04 Nov 2014 02:13:24 GMT"
 std::string rfc1123(std::time_t time);
 
+// YYYY-mm-dd HH:MM:SS e.g. "2015-11-26 16:11:23"
+std::string iso8601(std::time_t time);
+
 }
 
 }