Proof? The HTC and MTC are both places where you enter and train for what seems like years at a time, only to leave and find that you've only been inside for a few weeks or months. They are both places where days pass like weeks, and weeks pass like days. Time is distorted and warped inside the MTC.
Here's another proof: In the HTC one of the people training remarks to his dad how much he hates the place, how he can't wait to get out, and so on and so forth. There are people who also do that in the MTC. However, when the person finally got out of the HTC, he was the most powerful character in the series, and in fact was the first one to ever go Super Sayan 2.