def elapsed_time(variable, time_stamp):
    """
    Simplifies the time axis, by subtracting the initial time.
    This is useful because usually the time stamps are given in a long format (i.e. in the order of 1e9)
    :param variable: Reference variable to obtain the size of the time array
    :param time_stamp: The time stamp array that is going to be simplified
    :return: Simplified time as Elapsed Time
    """
    elapsed = [None] * len(variable)
    for i in range(len(variable)):
        elapsed[i] = time_stamp[i] - time_stamp[0]
    return elapsed