add the time as a string

AircraftSchedule.proto

@@ -8,5 +8,5 @@ message AircraftSchedule {
     Aircraft aircraft       = 1; // aircraft information
     string   arrivalRoute   = 2; // planned arrival route
     string   arrivalRunway  = 3; // planned arrival runway
-    Time     reachIafAtTime = 4; // UTC time when the aircraft needs to pass IAF
+    string   reachIafAtTime = 4; // UTC time when the aircraft needs to pass IAF
 }