what is the meaning of linearly time invarient system?

Answer 1

A LTI system is one where the following holds true:

if Xi is an input and Yi is an output
f(x1) = Y1
f(x2) = Y2
f(x1+x2) = y1+y2
and
f(A*x1+B*x2) = A*y1+B*y2
basically if the input is a sum of two signals then the output is a sum of the two individual system responses to the individual inputs. And scaling the inputs scales the outputs by the same constant (my A's and B's)
That's the linear bit.

As for time invariance basically this means that if you shift the input signal in time the output signal shifts by an equal amount.
f(t) = Y(t)
f(t-tau) = Y(t-tau)
i.e. the system response is invariant to a time shift.

Answer 2

Total time does not depend on linear attributes.