In time series charts (where the x-axis is time) you can hover over a data point to see more information, including the time that corresponds to that data point. This time label has different meanings depending on the type of chart.
For synthetic charts, each data point represents one or more test results. The time label might say "Tue 22 May 12:00 NZST". This is the time that the synthetic tests were scheduled to execute. In most cases the actual test time is later than that, because tests are queued up and executed in order. To find the actual time that the test was executed, click on the data point and select "View Test" and then "WebPageTest Results".
For LUX charts, the time label might look like "Sat 26 May 09:00 NZST". This is the value for the end of the interval. To find the beginning of the interval, hover over the next data point to the left. It might say "Sat 26 May 08:50 NZST". This means the data point represents the data from 08:50 to 09:00.