Add AZFD0015 diagnostic event documentation

Author: brettsam

articles/azure-functions/TOC.yml

@@ -1060,6 +1060,8 @@
         href: errors-diagnostics/diagnostic-events/azfd0012.md
       - name: AZFD0013
         href: errors-diagnostics/diagnostic-events/azfd0013.md
+      - name: AZFD0015
+        href: errors-diagnostics/diagnostic-events/azfd0015.md
   - name: host.json 2.x reference
     href: functions-host-json.md
   - name: host.json 1.x reference

articles/azure-functions/errors-diagnostics/diagnostic-events/azfd0015.md

@@ -0,0 +1,38 @@
+---
+title: "AZFD0015: Timer trigger schedule is not a CRON expression"
+titleSuffix: "Azure Functions"
+description: "Learn how to troubleshoot the event 'AZFD0015: Timer trigger schedule is not a CRON expression' in Azure Functions."
+ms.topic: error-reference
+ms.date: 02/10/2026
+
+---
+
+# AZFD0015: Timer trigger schedule is not a CRON expression
+
+This event occurs when a timer trigger is configured with a non-CRON schedule expression (such as a `TimeSpan`) on a Consumption plan where the scale controller only supports CRON expressions.
+
+| | Value |
+|-|-|
+| **Event ID** |AZFD0015|
+| **Severity** |Warning|
+
+## Event description
+
+Azure Functions timer triggers support both [CRON expressions][cron] and constant interval expressions (such as `TimeSpan` values like `"00:05:00"`). However, the scale controller used by Consumption plans only understands CRON expressions. When a non-CRON schedule is used on these plans, the timer trigger starts and fires correctly, but the scale controller can't interpret the schedule. This may lead to unexpected scaling behavior.
+
+This warning currently applies to Linux Consumption plans. On Windows Consumption and Flex Consumption plans, non-CRON expressions are blocked entirely and produce a startup error.
+
+## How to resolve the event
+
+Replace your timer trigger's schedule expression with an equivalent CRON expression. CRON expressions can use either 5 fields (minute granularity) or 6 fields (second granularity).
+
+For example, instead of a `TimeSpan` of `"00:05:00"` (every 5 minutes), use the CRON expression `"0 */5 * * * *"` (6-digit) or `"*/5 * * * *"` (5-digit).
+
+For more information on CRON expressions in Azure Functions, see [Timer trigger for Azure Functions][timer-trigger].
+
+## When to suppress the event
+
+This event shouldn't be suppressed. While the timer fires correctly today, the scale controller can't interpret non-CRON schedules, which may result in scaling issues. Migrating to a CRON expression ensures correct behavior.
+
+[cron]: /azure/azure-functions/functions-bindings-timer#ncrontab-expressions
+[timer-trigger]: /azure/azure-functions/functions-bindings-timer