'X' Modulation means that 'X' is changing. Amplitude, Frequency, Phase, Pulse Width, etc. Some parameter of the signal. I'm not sure how you'd change Time itself.
There's a Pulse Position Modulation, which is changing the relative timing of a pulse, typically relative to a reference pulse. A bit like ignition advance relative to TDC in a spark ignition engine. Rarely used in radio communications, might be a bit more common in radio navigation.
You can easily Google Pulse Width Modulation or PWM. I assume Wiki would have a good explanation.
