Simple basics of timekeeping in ESXi and virtual machines

[garryking]

Hello there,

I was playing with ESXi and virtual machines in my home lab and quite confused with how exactly time is maintained in ESXi and virtual machines running with various guest OS on it. I understand this concept for many is probably very straight forward and not a rocket science but as I am quite new to VMware and virtualisation as such, I am finding it bit difficult to grasp.

I have gone through few different KB's and timekeeping mechanisms but I want to know from other sys admins or experts in their simple words how does time keeping exactly works, what is the real role of NTP server and VMware tools? One thing I noticed is when you run date command in your via SSH session on your host it displays time in UTC. On the other hand when you see this same host via VI client from configuration-->time configuration the time is different. How does ESXi host maintain and show 2 different timings? Also I believe one of the reason that Data & Time is shown in red is as it's incorrect but than how does ESXi host know that it's incorrect when I haven't configured any NTP servers? Is ESXi host comparing time with what's in BIOS? As soon as I correct the time manually to reflect my local time it doesn't show in red colour any more? If ESXi host can maintain it's own time than what is the use case for configuring NTP server like pool.ntp.org??

Can someone please explain how this all works in simple terms...thank you

[a_p_]

An ESXi host only maintains the UTC time, and what you see in the vSphere Client is translated into the client's time zone. A red date/time indicates that the vSphere Client detected a gap between its own time and the time on the ESXi host. As a best practice, configure the ESXi host with a reliable time server (this time server must not be a virtual machine running on the ESXi host!). For virtual machines in an AD domain, the VMs will synchronize with the domain controllers, so only configure time synchronization VMware Tools if a VM is not member of a domain and doesn't use an NTP for time synchronization.

André