From 7b35c3036e0b2e918f176473d40908ac7071ebf3 Mon Sep 17 00:00:00 2001
From: Nerdix <70015952+N3rdix@users.noreply.github.com>
Date: Tue, 3 Sep 2024 19:47:00 +0200
Subject: [PATCH] Enhance error handling when changing a timer's duration
 (#121786)

* Update remaining before checking duration

* fix comment

* calculation based on transient field

* lint

* remove useless brackets
---
 homeassistant/components/timer/__init__.py | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/homeassistant/components/timer/__init__.py b/homeassistant/components/timer/__init__.py
index c2057551239..19b1de427ef 100644
--- a/homeassistant/components/timer/__init__.py
+++ b/homeassistant/components/timer/__init__.py
@@ -338,7 +338,9 @@ class Timer(collection.CollectionEntity, RestoreEntity):
             raise HomeAssistantError(
                 f"Timer {self.entity_id} is not running, only active timers can be changed"
             )
-        if self._remaining and (self._remaining + duration) > self._running_duration:
+        # Check against new remaining time before checking boundaries
+        new_remaining = (self._end + duration) - dt_util.utcnow().replace(microsecond=0)
+        if self._remaining and new_remaining > self._running_duration:
             raise HomeAssistantError(
                 f"Not possible to change timer {self.entity_id} beyond duration"
             )
@@ -349,7 +351,7 @@ class Timer(collection.CollectionEntity, RestoreEntity):
 
         self._listener()
         self._end += duration
-        self._remaining = self._end - dt_util.utcnow().replace(microsecond=0)
+        self._remaining = new_remaining
         self.async_write_ha_state()
         self.hass.bus.async_fire(EVENT_TIMER_CHANGED, {ATTR_ENTITY_ID: self.entity_id})
         self._listener = async_track_point_in_utc_time(