So if you open a message at 10 am, it's time stamp changes to 10 am? And if you open it again at 11.30, the timestamp changes to 11.30?
That doesn't sound very likely. What software are you using to look at your GMail account? Are you using Safari, or Apple Mail? You chose not to give that rather important piece of info. And are you also reading your mail on an iPhone or an iPad?
The timestamp on a message in your inbox is the time it was RECEIVED by you not the time it was sent. You can find the time it was sent by looking at Show Original under the Reply dropdown.