This is one for those of you who live in the UK, although it could be relevant to anyone. Remember that at various times in the year we are one hour shy of UTC because we operate on BST (British Summer Time) not UTC/GMT. This is where for daylight saving reasons the clocks go forwards and backwards during the year. So, sometimes we are in sync with UTC/GMT and other times we are not.
You can see this difference between BST and UTC during the summer months when we are, in the UK, 1hr behind UTC when you run the tzselect utility.
During the Winter when the clocks go back 1hr in the UK for Daylight Savings we catch up with UTC/GMT.
Therefore, it might be best not to use X System clock, which uses UTC. If you do, your server will be forever +1 or -1 BST. If you do send your machine into the past, the system availability report will tell you someone has manipulated the time. Under warnings you would see “Time went backwards. Reported downtime durations may be inaccurate”.
I only realized that I had been setting my time inaccurately in ESX this last week when I was sitting on the VMware course! The bizarre thing is that I hadn’t spotted it sooner!
For more information on this subject see:
This was first published in November 2005