From 84b112024fd252a98e1f41a7a7e4143ae0f57262 Mon Sep 17 00:00:00 2001
From: Philip Allgaier <mail@spacegaier.de>
Date: Sat, 23 Jan 2021 14:37:56 +0100
Subject: [PATCH] Note about isoformat() for frontend templates (#16097)

Co-authored-by: Franck Nijhof <git@frenck.dev>
---
 .../_docs/configuration/templating.markdown   | 26 ++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/source/_docs/configuration/templating.markdown b/source/_docs/configuration/templating.markdown
index a9f94e3c5da..80b9298657f 100644
--- a/source/_docs/configuration/templating.markdown
+++ b/source/_docs/configuration/templating.markdown
@@ -195,7 +195,31 @@ The same thing can also be expressed as a filter:
 - Filter `timestamp_utc` converts a UNIX timestamp to its string representation representation as date/time in UTC timezone.
 - Filter `timestamp_custom(format_string, local_time=True)` converts an UNIX timestamp to its string representation based on a custom format, the use of a local timezone is default. Supports the standard [Python time formatting options](https://docs.python.org/3/library/time.html#time.strftime).  
 
-Note: [UNIX timestamp](https://en.wikipedia.org/wiki/Unix_time) is the number of seconds that have elapsed since 00:00:00 UTC on 1 January 1970. Therefore, if used as a function's argument, it can be substituted with a numeric value (`int` or `float`):  
+<div class='note'>
+	
+[UNIX timestamp](https://en.wikipedia.org/wiki/Unix_time) is the number of seconds that have elapsed since 00:00:00 UTC on 1 January 1970. Therefore, if used as a function's argument, it can be substituted with a numeric value (`int` or `float`).
+
+</div>
+
+<div class='note warning'>
+	
+If your template is returning a timestamp that should be displayed in the frontend (e.g., as a sensor entity with `device_class: timestamp`), you have to ensure that it is the ISO 8601 format (meaning it has the "T" separator between the date and time portion). Otherwise, frontend rendering on macOS and iOS devices will show an error. The following value template would result in such an error:
+
+{% raw %}
+
+`{{ states.sun.sun.last_changed }}` => `2021-01-24 07:06:59+00:00` (missing "T" separator)
+
+{% endraw %}
+
+To fix it, enforce the ISO conversion via `isoformat()`:
+
+{% raw %}
+
+`{{ states.sun.sun.last_changed.isoformat() }}` => `2021-01-24T07:06:59+00:00` (contains "T" separator)
+
+{% endraw %}
+
+</div>
 
 {% raw %}
 ```yaml