Resolving Time Discrepancies and UTC Offsets Between WinCC Alarm Logging and OS System Clocks

In SIMATIC PCS 7, time discrepancies where alarm timestamps in WinCC AlarmControl differ by one hour from the OS system clock (e.g., 8:33 AM alarm vs. 7:31 AM system time) are caused by mismatched time zone and daylight saving time (DST) interpretations between the automation system (AS) and the operator station (OS). In PCS 7, the AS CPU system clock runs in Coordinated Universal Time (UTC) and stamps ALARM_8P messages in UTC. If the CPU clock is set to local time, or if WinCC AlarmControl is configured to display raw PLC time without applying local time zone offsets, a 1-hour shift occurs. To resolve this, ensure plant-wide time synchronization is configured via NTP/SICLOCK with the AS operating in UTC, and set WinCC AlarmControl message blocks to display 'Local time'. For systems switching daylight saving, apply the Siemens registry parameter NotConsiderDaylight.

Executive Summary

Accurate sequence-of-events analysis in SIMATIC PCS 7 requires precise time stamping across all distributed automation systems and operator stations. A common fault encountered in PCS 7 V8.1/V8.2 occurs when the WinCC AlarmControl list displays alarm timestamps offset by exactly one hour from the Windows system clock displayed in the taskbar or graphic header. This discrepancy distorts incident investigations and alarm logging consistency. The root cause lies in misaligned time synchronization architectures, improper UTC-to-local conversions, or unhandled daylight saving time transitions. This guide provides the diagnostic steps to eliminate alarm timestamp offsets according to Siemens SIOS guidelines.

Architecture and Fundamental Mechanics

The SIMATIC PCS 7 time synchronization concept relies on a standardized reference framework:
1. The Automation System (AS) controller internal clock must always run in Coordinated Universal Time (UTC). Channel driver blocks and ALARM_8P message blocks acquire this raw CPU hardware timestamp at the millisecond level.
2. When ALARM_8P transmits an alarm telegram to the WinCC Alarm Logging subsystem over the Plant Bus, the payload contains the unadulterated UTC timestamp without local time zone or daylight saving indicators.
3. WinCC OS Runtime evaluates the raw timestamp against the Windows OS time zone settings. If WinCC AlarmControl is set to 'Local Time', it adds or subtracts the local UTC offset (e.g., UTC+1) and active Daylight Saving Time (+1 hour).

A 1-hour discrepancy manifests under specific misconfigurations:
- The AS clock was manually synchronized or set to Local Time instead of UTC, causing WinCC to apply the local time zone offset twice.
- WinCC AlarmControl column formatting for 'Date/Time' is set to display 'System time' (UTC) while the operator expects local wall-clock time.
- The central time master (e.g., SICLOCK TC 400 or NTP server) does not properly broadcast daylight saving transition flags, or the AS handles daylight saving locally while WinCC also applies it.

Under Siemens FAQ 7604251, in environments where the AS automatically toggles between summer and winter time, WinCC's communication DLL (NrmS7pmc) can be instructed to ignore daylight saving using the DWORD registry key NotConsiderDaylight = 1.

Step-by-Step Implementation Procedure

Verify AS Controller Clock Configuration:
a. In SIMATIC Manager, connect online to the CPU 410-5H or S7-400 controller.
b. Select 'PLC -> Set Time of Day...'.
c. Verify whether the controller clock is configured to UTC. In a standard PCS 7 architecture, the AS MUST be set to UTC (not local time).
d. In HW Config, open CPU Properties -> 'Time-of-Day Synchronization' and verify that synchronization via Industrial Ethernet (Plant Bus) is set to 'As slave'.
Configure Central Time Master on Plant Bus:
a. Designate a central NTP time server (e.g., SICLOCK TC 100/400 or external network time server).
b. On the WinCC OS Server, open the WinCC Explorer 'Time Synchronization' editor.
c. Check 'Use time synchronization', select the Plant Bus network adapter, and enable 'Slave' mode (or 'Master' if the OS Server acts as the time master).
Configure WinCC AlarmControl Display Formatting:
a. In WinCC Graphics Designer, open the process graphic containing the alarm list (e.g., @AlarmNew.pdl or custom overview).
b. Open the Object Properties of WinCC AlarmControl.
c. Navigate to Message blocks -> Date/Time.
d. Under 'Time format', select 'Local time' instead of 'System time' or 'UTC'. This ensures WinCC dynamically converts raw UTC telegram stamps to local wall-clock time.
Apply Registry Fix for Daylight Saving Shifts (Siemens FAQ 7604251):
a. If the PLC broadcasts local timestamps that conflict with Windows DST adjustments, open Windows Registry Editor (regedit) on the OS Server.
b. Navigate to: HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Siemens\WinCC\NormDLL\NrmS7pmc.
c. Create a new DWORD (32-bit) value named NotConsiderDaylight and set its value to 1.
d. Restart WinCC Runtime to apply the normalized timestamp handling.

Verification

Generate an alarm in the AS (e.g., trip an analog high alarm on MonAnL or digital input on MonDiL).
In WinCC AlarmControl, compare the alarm incoming timestamp with the Windows taskbar clock: confirm both timestamps match within 1 second.
Open SIMATIC Manager online to the CPU and execute 'PLC -> Module Information -> Diagnostic Buffer': confirm internal event timestamps align precisely with UTC time.
Verify that historical alarms in WinCC Alarm Logging preserve correct sequential order across seasonal daylight saving transitions.

Limitations and Operational Precautions

Changing the CPU time configuration in HW Config (such as enabling NTP synchronization or switching time zones) may require a CPU STOP download if interface settings are modified. In redundant OS architectures, time synchronization parameters must be identically mirrored on both partner servers. Do not set both OS Servers and AS controllers as active time masters on the same subnet, as competing synchronization broadcasts cause clock jitter and erratic message sequencing.

Engineer FAQ

Why does the alarm list show a 1-hour difference between row timestamp and screen header?

The alarm list is rendering the raw UTC timestamp generated by the PLC, while the screen header displays the local Windows time zone (e.g., UTC+1 during standard time or UTC+2 during daylight saving).

Should PCS 7 AS controllers be set to UTC or local time?

Siemens strictly mandates that all PCS 7 automation systems be configured to UTC to maintain seamless cross-timezone operations and prevent duplicate timestamps during daylight saving fall-back.

What does the registry key NotConsiderDaylight do?

It prevents the WinCC S7 communication normalization DLL (NrmS7pmc) from applying an extra daylight saving time correction to messages originating from the PLC.

Can time synchronization be configured without a hardware master like SICLOCK?

Yes. A designated domain controller, NTP server, or the primary WinCC OS Server can act as the central time master on the Plant Bus.

How does time stamping work with ET 200SP HA high-precision I/O?

High-precision digital input modules stamp events directly at the I/O terminal with 1 ms resolution using the AS-distributed clock before transmitting the stamped record to the CPU.