From 9fbf8ac3d9527181c5279dc6c6e9140f09e5b315 Mon Sep 17 00:00:00 2001
From: Andrey Smirnov <andrey.smirnov@siderolabs.com>
Date: Wed, 22 May 2024 15:05:59 +0400
Subject: [PATCH] fix: offset calculation for NTP responses

The problem here is that `time.Duration` and `time.Time` types don't
exactly match the NTP 64-bit time value, so the expected calculations
for big time jumps (including across NTP eras) don't work properly.

See:

* https://www.eecis.udel.edu/~mills/y2k.html
* https://www.eecis.udel.edu/~mills/time.html

Ref: https://github.com/siderolabs/talos/issues/8771

Signed-off-by: Andrey Smirnov <andrey.smirnov@siderolabs.com>
---
 ntp.go      | 15 ++++++++++++---
 ntp_test.go | 21 +++++++++++++++++++++
 2 files changed, 33 insertions(+), 3 deletions(-)

diff --git a/ntp.go b/ntp.go
index 0d25544..009528b 100644
--- a/ntp.go
+++ b/ntp.go
@@ -749,9 +749,18 @@ func rtt(org, rec, xmt, dst ntpTime) time.Duration {
 func offset(org, rec, xmt, dst ntpTime) time.Duration {
 	// local clock offset
 	//   offset = ((rec-org) + (xmt-dst)) / 2
-	a := rec.Time().Sub(org.Time())
-	b := xmt.Time().Sub(dst.Time())
-	return (a + b) / time.Duration(2)
+	//
+	// see	https://www.eecis.udel.edu/~mills/time.html for more details
+	// the input is 64 unsigned numbers, output is 63 bits signed (precision)
+	a := int64(rec) - int64(org)
+	b := int64(xmt) - int64(dst)
+	d := (a + b) / 2
+
+	if d > 0 {
+		return ntpTime(d).Duration()
+	} else {
+		return -ntpTime(-d).Duration()
+	}
 }
 
 func minError(org, rec, xmt, dst ntpTime) time.Duration {
diff --git a/ntp_test.go b/ntp_test.go
index badf383..bcb928d 100644
--- a/ntp_test.go
+++ b/ntp_test.go
@@ -13,6 +13,7 @@ import (
 	"time"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 // The NTP server to use for online unit tests. May be overridden by the
@@ -388,6 +389,26 @@ func TestOfflineOffsetCalculationNegative(t *testing.T) {
 	assert.Equal(t, expectedOffset, offset)
 }
 
+func TestOfflineOffsetCalculationNegativeBig(t *testing.T) {
+	// these timestamps are in different NTP epochs,
+	// see:
+	//  * https://www.eecis.udel.edu/~mills/y2k.html
+	serverNow, err := time.Parse("2006-01-02 15:04:05", "2024-05-22 11:30:15")
+	require.NoError(t, err)
+
+	clientNow, err := time.Parse("2006-01-02 15:04:05", "2047-01-01 00:00:00")
+	require.NoError(t, err)
+
+	org := toNtpTime(clientNow)
+	rec := toNtpTime(serverNow)
+	xmt := toNtpTime(serverNow.Add(1 * time.Second))
+	dst := toNtpTime(clientNow.Add(1 * time.Second))
+
+	expectedOffset := -clientNow.Sub(serverNow)
+	offset := offset(org, rec, xmt, dst)
+	assert.Equal(t, expectedOffset, offset)
+}
+
 func TestOfflineReferenceString(t *testing.T) {
 	cases := []struct {
 		Stratum byte