It is a basic geometry statement that between any two points on a line there is another point on that line . But , between any two instants of time , is there another instant of time ?

The physics of short time or short distance (Planck time or Planck distance) is very uncertain.

It is a basic geometry statement that between any two points on a line there is another point on that line . But , between any two instants of time , is there another instant of time ?

 

If you model time as a smooth line then yes.

 

In theories of classical and quantum mechanics and field theory time is continuous and smooth. So on the scales we see around us and on scales that we can probe, time seems continuous.

 

However, it maybe possible that space and time has a discrete nature on the scale of the Planck length. That is quantum gravity is expected to introduce some noncontinuous structure to space-time.