Revert "Revert "gs: Ignore stale tmms (#208)" (#209)" (#210)

This reverts commit 81a6290.

pkg/ttnpb/udp/translation.go

@@ -29,10 +29,11 @@ import (
 )
 
 const (
-	delta  = 0.001 // For GPS comparisons
-	lora   = "LORA"
-	fsk    = "FSK"
-	lrfhss = "LR-FHSS"
+	gpsTimeDelta = 5 * time.Second // for validating tmms/GPS time
+	delta        = 0.001           // For GPS comparisons
+	lora         = "LORA"
+	fsk          = "FSK"
+	lrfhss       = "LR-FHSS"
 
 	// eirpDelta is the delta between EIRP and ERP.
 	eirpDelta = 2.15
@@ -201,8 +202,15 @@ func convertUplink(rx RxPacket, md UpstreamMetadata) (*ttnpb.UplinkMessage, erro
 	var goTime, goGpsTime time.Time
 	switch {
 	case rx.Tmms != nil:
-		goGpsTime = gpstime.Parse(time.Duration(*rx.Tmms) * time.Millisecond)
-		goTime = goGpsTime
+
+		ggt := gpstime.Parse(time.Duration(*rx.Tmms) * time.Millisecond)
+		currentUTC := time.Now().UTC()
+		diff := ggt.Sub(currentUTC).Abs().Seconds()
+		if diff < gpsTimeDelta.Seconds() {
+			goGpsTime = ggt
+			goTime = goGpsTime
+		}
+
 	case rx.Time != nil:
 		goTime = time.Time(*rx.Time)
 	}