Fix for older glib versions

xmpp-vala/src/module/xep/0082_date_time_profiles.vala

@@ -1,12 +1,23 @@
 namespace Xmpp.Xep.DateTimeProfiles {
 
 public DateTime? parse_string(string time_string) {
-    return new DateTime.from_iso8601(time_string, null);
+    // TODO with glib >= 2.56
+    // return new DateTime.from_iso8601(time_string, null);
+
+    TimeVal time_val = TimeVal();
+    if (time_val.from_iso8601(time_string)) {
+        return new DateTime.from_unix_utc(time_val.tv_sec);
+    }
+    return null;
+
 }
 
 
 public string to_datetime(DateTime time) {
-    return time.to_utc().format_iso8601().to_string();
+    // TODO with glib >= 2.62
+    // return time.to_utc().format_iso8601().to_string();
+
+    return time.to_utc().format("%Y-%m-%dT%H:%M:%SZ");
 }
 
 }