Format timestamps using ISO 8601 formatting (e.g. "2018-02-28T12:34:56Z")

* Z is the zone designator for the zero UTC offset.
* T is the delimiter used to separate date and time.

This makes it clear for the end-user that the date/time logged is
specified in UTC and not in the local time zone.

src/utiltime.cpp

@@ -85,3 +85,15 @@ std::string DateTimeStrFormat(const char* pszFormat, int64_t nTime)
     ss << boost::posix_time::from_time_t(nTime);
     return ss.str();
 }
+
+std::string FormatISO8601DateTime(int64_t nTime) {
+    return DateTimeStrFormat("%Y-%m-%dT%H:%M:%SZ", nTime);
+}
+
+std::string FormatISO8601Date(int64_t nTime) {
+    return DateTimeStrFormat("%Y-%m-%d", nTime);
+}
+
+std::string FormatISO8601Time(int64_t nTime) {
+    return DateTimeStrFormat("%H:%M:%SZ", nTime);
+}