Session hours start at the time that the session starts. In the case of the SPX, the first hour bar would start at 9:30 AM and close at 10:30 AM.
If your chart’s bar building is set one way, and someone else’s charts are set another way, then there’s a good chance that they won’t align. You'd probably agree that a bar that shows activity from 9:30 AM to 10:00 AM could look quite different from one that ran from 9:30 AM to 10:30 AM.
So if you’re not seeing similar bars in your charts, check that you’re using the same bar building type as your trading room host. It’s usually Session Hours. The setting can be found in the Settings tab of the Format/Symbols dialog, as shown here:
This can make a significant difference in your charts. For example, here's a chart with session hours enabled, where the bars end on the 30-minute mark. Note there's no red finger at any point on 24 July:
If we switch to natural hours, where the bars end at the top of the hour, we get a red finger on the same day:
So it's very important to be aware of your bar building hours setting if you want to match the charts in the room or someone else's chart.