AP_RTC: Don't allow RTC times before 2019

libraries/AP_RTC/AP_RTC.cpp

@@ -33,6 +33,8 @@ const AP_Param::GroupInfo AP_RTC::var_info[] = {
 
 void AP_RTC::set_utc_usec(uint64_t time_utc_usec, source_type type)
 {
+    const uint64_t oldest_acceptable_date = 1546300800000; // 2019-01-01 0:00
+
     if (type >= rtc_source_type) {
         // e.g. system-time message when we've been set by the GPS
         return;
@@ -43,6 +45,11 @@ void AP_RTC::set_utc_usec(uint64_t time_utc_usec, source_type type)
         return;
     }
 
+    // don't allow old times
+    if (time_utc_usec < oldest_acceptable_date) {
+        return;
+    }
+
     const uint64_t now = AP_HAL::micros64();
     const int64_t tmp = int64_t(time_utc_usec) - int64_t(now);
     if (tmp < rtc_shift) {