Think about it, say we have dt. dt is an infinitesimal amount of time that elapses with respect to another quantity. However, physics sets standards for how we define time. The shortest length of time possible is planck time. Planck time is 10^-43 seconds in magnitude and is defined as the shortest meaningful time in the universe. Any time smaller than this makes no physical sense, so there is no such thing as an infinitesimal amount of time. So does dt automatically equal 10^-43 seconds or what?