To measure time, we need a clock. We all have our own biological clock, and that’s what we normally use. However, when we want to be precise, we build ourselves an instrument.
The way a clock works is that it takes something that moves at a predictable speed and make it give off a tick every time it has moved a precise distance. The smallest possible time unit we can register is in other words a function of the smallest possible ruler and the fastest possible speed.
The smallest possible time unit is therefore the time it takes a photon to cross an electron. If something happens faster than this, the time laps cannot be registered in any way.
An instantaneous event is anything that happens faster than it takes a photon to cross an electron.