Understanding the actual uptime of a virtual machine (as seen inside the guest OS) and the uptime displayed in the vCenter UI.

Products

• VMware vCenter Server

Issue/Introduction

The guest OS uptime reflects how long the operating system inside the VM has been running since its last reboot. This is visible from within the OS itself.

The vCenter UI uptime, however, shows how long the VM has been powered on at the hypervisor level.

• If the guest OS is rebooted without powering off the VM, the vCenter uptime will remain unchanged.
• If the VM is reset, the  vCenter uptime will remain unchanged.
• If the VM is vMotioned to another host, the vCenter uptime will be changed. 
• If the VM is powered off and then powered on, the vCenter uptime will be changed. 

This often leads to a mismatch between the two values.

Understanding the difference is important for accurately diagnosing reboots or availability issues.

Environment

vSphere 7.x, 8.x 

Cause

vCenter Uptime Reflects VMX Runtime, Not Guest OS Metrics

vCenter uses the uptimeSeconds value from the VM summary, which tracks the duration the VMX process has been active (i.e., since the VM was powered on or vMotioned). This is not linked to the actual uptime from inside the guest OS.

Resolution

This is expected behavior by design. If you require visibility into actual guest OS uptime:

• Use OS-level tools (e.g., uptime, who -b, or systeminfo) within the guest.
• Or integrate with OS monitoring tools (e.g., vRealize Operations, Nagios, Zabbix, Prometheus with guest agents).