From 8c7fa05d6b77c890e01950acca4516bf8e18f326 Mon Sep 17 00:00:00 2001
From: Denys Smirnov <dennwc@pm.me>
Date: Mon, 6 Oct 2025 20:38:59 +0300
Subject: [PATCH] Document mono package, support Unix and Parse.

---
 .changeset/mono-docs-and-api.md |  5 +++
 utils/mono/mono.go              | 54 ++++++++++++++++++++++++++++++++-
 utils/mono/mono_test.go         | 17 ++++++++++-
 3 files changed, 74 insertions(+), 2 deletions(-)
 create mode 100644 .changeset/mono-docs-and-api.md

diff --git a/.changeset/mono-docs-and-api.md b/.changeset/mono-docs-and-api.md
new file mode 100644
index 000000000..a0082953b
--- /dev/null
+++ b/.changeset/mono-docs-and-api.md
@@ -0,0 +1,5 @@
+---
+"@livekit/protocol": patch
+---
+
+Document mono package, support Unix and Parse.
diff --git a/utils/mono/mono.go b/utils/mono/mono.go
index eab94df71..cfba51650 100644
--- a/utils/mono/mono.go
+++ b/utils/mono/mono.go
@@ -12,6 +12,16 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// Package mono enforces use of monotonic time when creating/parsing time.Time from external sources.
+//
+// Using time.Now produces monotonic time values that correctly measure time difference in the presence of clock resets.
+//
+// On the other hand, time produce by time.Unix or time.Parse doesn't have this property. Clock reset may lead to incorrect
+// durations computed from these timestamps. To fix this, prefer using Unix and Parse provided by this package.
+//
+// Monotonic time could also be erased when using functions like Truncate, Round, In, UTC. Be careful when using these.
+//
+// More details: https://go.googlesource.com/proposal/+/master/design/12914-monotonic.md
 package mono
 
 import "time"
@@ -21,21 +31,63 @@ var (
 	epochNano = epoch.UnixNano()
 )
 
+// resetClock resets the reference timestamp.
+// Used in tests only.
+func resetClock() {
+	epoch = time.Now()
+	epochNano = epoch.UnixNano()
+}
+
+// jumpClock adjusts reference timestamp by a given duration emulating a clock reset/jump.
+// Used in tests only.
+func jumpClock(dt time.Duration) {
+	epoch = epoch.Add(-dt) // we pretend time.Now() jumps, not the reference
+	epochNano = epoch.UnixNano()
+}
+
+// FromTime ensures that time.Time value uses monotonic clock.
+//
+// Deprecated: You should probably use Unix or Parse instead.
 func FromTime(t time.Time) time.Time {
+	return fromTime(t)
+}
+
+func fromTime(t time.Time) time.Time {
 	if t.IsZero() {
 		return time.Time{}
 	}
 	return epoch.Add(t.Sub(epoch))
 }
 
+// Now is a wrapper for time.Time.
+//
+// Deprecated: time.Now always uses monotonic clock.
 func Now() time.Time {
-	return epoch.Add(time.Since(epoch))
+	return time.Now()
+}
+
+// Unix is an analog of time.Unix that produces monotonic time.
+func Unix(sec, nsec int64) time.Time {
+	return fromTime(time.Unix(sec, nsec))
+}
+
+// Parse is an analog of time.Parse that produces monotonic time.
+func Parse(layout, value string) (time.Time, error) {
+	t, err := time.Parse(layout, value)
+	if err != nil {
+		return time.Time{}, err
+	}
+	return fromTime(t), nil
 }
 
+// UnixNano returns the number of nanoseconds elapsed, based on the application start time.
+// This value may be different from time.Now().UnixNano() in the presence of time resets.
 func UnixNano() int64 {
 	return epochNano + int64(time.Since(epoch))
 }
 
+// UnixMicro returns the number of microseconds elapsed, based on the application start time.
+// This value may be different from time.Now().UnixMicro() in the presence of time resets.
 func UnixMicro() int64 {
 	return UnixNano() / 1000
 }
diff --git a/utils/mono/mono_test.go b/utils/mono/mono_test.go
index a3a3c4b6e..2e89ce94a 100644
--- a/utils/mono/mono_test.go
+++ b/utils/mono/mono_test.go
@@ -9,9 +9,24 @@ import (
 
 func TestMonoZero(t *testing.T) {
 	ts := time.Time{}
-	ts2 := FromTime(ts)
+	ts2 := fromTime(ts)
 	require.True(t, ts.IsZero())
 	require.True(t, ts2.IsZero())
 	require.True(t, ts.Equal(ts2))
 	require.Equal(t, ts.String(), ts2.String())
 }
+
+func TestMono(t *testing.T) {
+	t.Cleanup(resetClock) // restore
+
+	ts1 := time.Now()
+	ts2 := ts1.Add(time.Second)
+
+	ts1m := fromTime(ts1)
+	// emulate a clock reset, +1h jump
+	// TODO: use synctest when we switch to Go 1.25
+	jumpClock(time.Hour)
+	ts2m := fromTime(ts2)
+
+	require.Equal(t, ts2.Sub(ts1), ts2m.Sub(ts1m))
+}