You want to know how many VMs have been built in your environment over the last month (or 30 days). We used to do this by looking at the Total VMs metric on the vSphere World object and do some calculations, like this.
As you'll notice we are using the Expression Transformation to calculate the {last - first) value of that metric over that time period (month).
This will give you monthly totals (or whatever period you'd chosen).
While this works, I'd like to see the VMs built in the last 30 days, which is a bit more difficult. But, Operations 8.18 made this much easier! Specifically, the ability to use the Timestamp Transformation in View filters. It's detailed here in the Operations 8.18 Release Notes.
Let's explore, my View looks like this.
I've captured the properties I want and now just need to filter on the last 30 days of VMs, which I can now do like this.
I now have details about the VMs created in the last 30 days.
If you want this Dashboard (and Views), you can find them here, enjoy!