The Heaviside Function, H(t) is defined as

H(t) =

1 if t>=0

0 if t<0

It is used in the study of electric circuits to represent the sudden surge of electric current, or voltage, when a switch is instantaneously turned on

My student asked me the following question,

"since time cannot be negative, then why the function is defined for negative values of t?"

What is the best explanation? is it because usually we want to have a function that is defined for all real numbers?

Thanks!