diff --git a/epan/proto.c b/epan/proto.c
index 04cacb61a7..95fcbc3b96 100644
--- a/epan/proto.c
+++ b/epan/proto.c
@@ -2600,11 +2600,11 @@ get_time_value(proto_tree *tree, tvbuff_t *tvb, const int start,
 
 			if (length == 8) {
 				tmp64secs  = tvb_get_ntoh64(tvb, start);
-				time_stamp->secs = (time_t)(int64_t)(tmp64secs - EPOCH_DELTA_2000_01_01_00_00_00_UTC);
+				time_stamp->secs = (time_t)(int64_t)(tmp64secs + EPOCH_DELTA_2000_01_01_00_00_00_UTC);
 				time_stamp->nsecs = 0;
 			} else if (length == 4) {
 				tmpsecs  = tvb_get_ntohl(tvb, start);
-				time_stamp->secs = (time_t)(int32_t)(tmpsecs - EPOCH_DELTA_2000_01_01_00_00_00_UTC);
+				time_stamp->secs = (time_t)(tmpsecs + EPOCH_DELTA_2000_01_01_00_00_00_UTC);
 				time_stamp->nsecs = 0;
 			} else {
 				time_stamp->secs  = 0;
@@ -2622,11 +2622,11 @@ get_time_value(proto_tree *tree, tvbuff_t *tvb, const int start,
 
 			if (length == 8) {
 				tmp64secs  = tvb_get_letoh64(tvb, start);
-				time_stamp->secs = (time_t)(int64_t)(tmp64secs - EPOCH_DELTA_2000_01_01_00_00_00_UTC);
+				time_stamp->secs = (time_t)(int64_t)(tmp64secs + EPOCH_DELTA_2000_01_01_00_00_00_UTC);
 				time_stamp->nsecs = 0;
 			} else if (length == 4) {
 				tmpsecs  = tvb_get_letohl(tvb, start);
-				time_stamp->secs = (time_t)(int32_t)(tmpsecs - EPOCH_DELTA_2000_01_01_00_00_00_UTC);
+				time_stamp->secs = (time_t)(tmpsecs + EPOCH_DELTA_2000_01_01_00_00_00_UTC);
 				time_stamp->nsecs = 0;
 			} else {
 				time_stamp->secs  = 0;
diff --git a/wsutil/epochs.h b/wsutil/epochs.h
index 27a884737e..c49e98c019 100644
--- a/wsutil/epochs.h
+++ b/wsutil/epochs.h
@@ -18,6 +18,12 @@
  * Deltas between the epochs for various non-UN*X time stamp formats and
  * the January 1, 1970, 00:00:00 (proleptic?) UTC epoch for the UN*X time
  * stamp format.
+ *
+ * XXX - All of these are unsigned, but note that most of these epochs are
+ * before the UN*X epoch but one is afterwards, which changes whether the
+ * delta should be subtracted or added to a time in the other epoch to
+ * produce a time_t in the UN*X epoch. Perhaps they should be signed,
+ * or there should macros that do the arithmetic and return a time_t?
  */
 
 /*
@@ -67,7 +73,10 @@
 /*
  * 2000-01-01 00:00:00 UTC.
  * Used by the Zigbee Zigbee Cluster Library protocol.
+ *
+ * This is *after* the UN*X epoch, so to convert from a time in seconds from
+ * this epoch this delta needs to be *added*, unlike the other values.
  */
-#define EPOCH_DELTA_2000_01_01_00_00_00_UTC ((unsigned)(((3*365 + 366)*7 + 2*365)*24*3600))
+#define EPOCH_DELTA_2000_01_01_00_00_00_UTC UINT64_C(((3*365 + 366)*7 + 2*365)*24*3600)
 
 #endif /* __EPOCHS_H__ */