Samples
The graph you make in a Visual Analytic is rarely only run once. When being used on live or historical data, it will be run many times - each time with different data and a different timestamp.
This means that there will be small spaces in time between each time your graph is 'run'. This is called the sample time.
In many cases, your sample time will be 1 second. However, this isn't guaranteed to be true.
You can choose to run analytics at higher or lower sample rates, or in some cases the available data will be coarse (ie. only be available with 10 second updates).
If you need logic that uses the amount of elapsed time, there are timing nodes that can help.