Another technique for animating time is to use a Speed Control strip in the Video Sequence editor.

Whenever you want to use the VSE it is **highly** recommended that you create a separate scene that is only the VSE and has no 3D objects in it. While it is not necessary, keeping VSE work separate from 3D work precludes a wide variety of very confusing mistakes.

Add a VSE strip referencing the target scene. Then add an Effect Strip / Speed. Then go into the properties of the Speed strip and uncheck `Stretch to input strip length` and uncheck `Use as speed`. Now you have a `Frame number` property which you can keyframe.
![speed control][1]

One of my favorite tricks is to loop the source track by creating a linear animation of the frame number and using the fcurves editor to add a `Cycles` modifier to turn it into a sawtooth.

The biggest limitation of the speed control strip is that when it samples the source scene, the frame number is turned into an integer (I'm not sure if it is rounded up or down, or nearest), so you can not use it to sample a 30 FPS source scene at 120 Hz (I tried and failed). You'll just get 4 copies of each discrete frame. You'll have to create the source scene at a high frame rate and keyframe it appropriately.

To be fair: the concept of continuous time used by SVG requires some heavy conceptual baggage that blender isn't prepared to address.

 [1]: https://i.sstatic.net/tzKV2.png