I finally found it!

I had been searching for this film for AGES because it had one of the more influential time travel scripts for it's time. All sorts of TV shows borrowed in some way from it, including:

The Outer Limits -

The Galaxy Being (slight, but too similar to be completely ignored)
Controlled Experiment (the ability to manipulate time and view the local result)
The Premonition (co-written by Melchior)

Star Trek -

The City on the Edge of Forever (time portal, view time on a screen)
Wink of an Eye (humans existing at a different time speed)
All Our Yesterdays (time portal, the ability to view time)

The Time Tunnel - concept

A FAR more intriguing concept of time and travel than most give it credit for, and still one of the most interesting even today.