You can use Piecewise function to create what you want.
For example, if you want that your parameter a is equal to 0 for 0<=t<6, and equal to 5 for t>=6, where t is indipendent variable representing time, you must write it:
In this case, t is not in your defined domain, it assumes value 0. If you want to specify another value for the rest of the range of t, you must specify it as second argument of piecewise. For example, if you want that in the rest of reals, a is equal to 3, you must write:
Then use it in your equation. I hope that it can help you.