diff --git a/src/tetra_tdma.c b/src/tetra_tdma.c
index 8fffa39..e138f49 100644
--- a/src/tetra_tdma.c
+++ b/src/tetra_tdma.c
@@ -95,6 +95,5 @@ char *tetra_tdma_time_dump(const struct tetra_tdma_time *tm)
 
 uint32_t tetra_tdma_time2fn(struct tetra_tdma_time *tm)
 {
-	/* FIXME: add hyperframe number !! */
-	return (tm->mn *18) + tm->fn;
+	return (((tm->hn * 60) + tm->mn) * 18) + tm->fn;
 }
diff --git a/src/tetra_tdma.h b/src/tetra_tdma.h
index 86c17af..01519ee 100644
--- a/src/tetra_tdma.h
+++ b/src/tetra_tdma.h
@@ -4,6 +4,7 @@
 #include <stdint.h>
 
 struct tetra_tdma_time {
+	uint16_t hn;    /* hyperframe number (1 ... 65535) */
 	uint32_t sn;	/* symbol number (1 ... 255) */
 	uint32_t tn;	/* timeslot number (1 .. 4) */
 	uint32_t fn;	/* frame number (1 .. 18) */