Hi @andreas-doehler
I double checked the problem.
I’m monitoring the vcenter using the special vsphere datasource and a installed checkmk agent. I think that’s the newer way. I guess. 
This is the rule:
The vsphere datasource outputs the system time. The time section is not coming from the agent. I used another vcenter host for comparison.
The execution time is still around 30 sec. The system time section is from the vsphere datasource. Because of the execution time, the reported system time from the vsphere datasource is outdated and i get a false positive in the system time check.
To workaround this I installed a plugin on the agent that takes the unix system time and converts it to the system time output like a windows system would output it to the checkmk. With this in place, I overwrite the wrong time from the vsphere datasource. But it’s not consistent and a pretty dirty workaround.
The optimal solution would be that the execution time is considered in the server side check for the system time.
Thanks for the help.