Add dates to RELNOTES.md for iperf-3.15-mt-beta1.
diff --git a/RELNOTES.md b/RELNOTES.md
index 94ede52..0a26d5b 100644
--- a/RELNOTES.md
+++ b/RELNOTES.md
@@ -1,8 +1,8 @@
 iperf3 Release Notes
 ====================
 
-iperf-3.15-mt-beta1
--------------------
+iperf-3.15-mt-beta1 2023-09-29
+------------------------------
 Accumulated release notes from iperf-3.14 and earlier multithreaded
 beta releases: