Resolving Two-Hour Time Axis Offsets in WinCC Online Trend Control and CEMAT Faceplates

In SIMATIC PCS 7 and CEMAT installations, an issue where the time coordinate axis on WinCC Online Trend Control faceplates displays two hours behind local wall-clock time is caused by a time base mismatch between UTC archiving and runtime display settings. SIMATIC PCS 7 controllers (S7-400 / CP 443-1) synchronize and log process data in Coordinated Universal Time (UTC). If the WinCC OS project or Trend Control is configured to display UTC on the time axis rather than the local time zone, regions operating at UTC+2 (such as Central European Summer Time) will show trends shifted exactly two hours back. To resolve this, open WinCC Explorer Project Properties, navigate to the 'Time Base' tab, and configure the runtime time display to 'Local computer time'.

Executive Summary

Process operators monitoring analog trends in WinCC or CEMAT faceplates frequently complain that the trend cursor and time axis display an incorrect time—typically showing a curve ending two hours prior to the current workstation clock. This temporal shift often triggers concern that communication to the AS controller has failed or that the Tag Logging archive has frozen. In reality, process data is actively logging, but the trend coordinate axis is rendering raw UTC timestamps without converting them to local workstation time. This guide explains the time synchronization architecture in SIMATIC PCS 7 and details the configuration steps to eliminate time axis offsets.

Architecture and Fundamental Mechanics

The PCS 7 time synchronization and archiving framework enforces global temporal consistency across all plant nodes:
1. The UTC Plant Standard:
To avoid ambiguous data overlaps during daylight saving time (DST) transitions, all PCS 7 AS controllers, CP 443-1 processors, and WinCC Tag Logging databases record and exchange data strictly in Coordinated Universal Time (UTC).
2. Tag Logging Storage vs. Runtime Presentation:
- Storage Layer: Every analog archive sample stored in Microsoft SQL Server is stamped with its exact UTC timestamp.
- Visualization Layer (WinCC Online Trend Control): When the trend control queries historical records, it can display the time coordinate axis in either of two modes:
- Mode A: Coordinated Universal Time (UTC) — useful for international operations spanning multiple time zones.
- Mode B: Local computer time — converts the UTC timestamp into the local time zone of the operator workstation, applying active daylight saving offsets.
3. Root Cause of the Two-Hour Shift:
In Central Europe or regions operating at UTC+2 (e.g., UTC+1 standard time + 1 hour summer time = UTC+2), if the WinCC Project Properties or the CEMAT trend template picture (@PG_..._TREND.PDL) specifies UTC as the time base, a live sample acquired at 14:00 local time (12:00 UTC) is rendered at '12:00' on the trend axis, creating an apparent 2-hour latency.

Step-by-Step Implementation Procedure

Verify AS Controller Time-of-Day Synchronization in HW Config:
a. Open STEP 7 HW Config for the S7-400 station.
b. Open Properties of the Industrial Ethernet CP card (e.g., CP 443-1) -> 'Time-of-Day Synchronization' tab.
c. Confirm that 'Enable time-of-day synchronization' is active via NTP or SIMATIC mode.
d. Verify that the AS clock is synchronized in UTC (no local time bias configured on the controller).
Configure Central Time Base in WinCC Explorer:
a. On the Engineering Station, open the OS project in WinCC Explorer.
b. Right-click the project root in the left tree and select 'Properties'.
c. In the Project Properties dialog, navigate to the 'Time Base' tab.
d. Under 'Time base for time display in runtime', change the setting from 'Coordinated Universal Time (UTC)' to 'Local computer time'.
e. Under 'Central time base for time inputs/outputs', ensure consistency across client and server stations.
f. Click OK to save changes.
Verify WinCC Online Trend Control Properties in Faceplate Templates:
a. In WinCC Explorer, open Graphics Designer.
b. If using standard APL or CEMAT faceplates, open the trend window template (e.g., @c_trend.pdl or @PG_APL_Trend.pdl).
c. Double-click the WinCC Online Trend Control object to open its Properties.
d. Navigate to the 'Time axis' tab.
e. Locate the parameter 'Time format' or 'Time zone': verify that it is set to 'Project setting' or 'Local computer time', rather than forced 'UTC'.
f. Save the picture.
Synchronize Redundant OS Servers and Clients:
a. Execute an OS compilation to ensure global project properties propagate.
b. Download project changes to the redundant OS servers.
c. Restart WinCC Runtime on operator client stations.

Verification

Open a process picture in WinCC Runtime and click a motor or analog transmitter to open its faceplate.
Switch to the 'Trends' tab.
Compare the rightmost edge of the trend curve with the digital clock displayed in the WinCC title bar:
- Confirm that the trend time axis aligns exactly with the current local wall-clock minute and second.
Check the CPU Diagnostic Buffer: verify that all logged events display synchronized timestamps.

Limitations and Operational Precautions

Ensure all OS clients, OS servers, and domain controllers synchronize to a common central NTP time server (such as SICLOCK); individual PC clock drift will cause trend curves to shift independently across different monitors.

Engineer FAQ

Why is my trend time two hours behind local time?

Because the AS archives data in UTC, and the WinCC trend display is configured to show raw UTC instead of converting to local time (UTC+2 for daylight saving regions).

Should the AS controller clock be set to local time or UTC?

The AS controller must always be set to UTC in SIMATIC PCS 7 to ensure unambiguous historical archiving and prevent data overwrites during daylight saving changes.