  A long standing problem in kinematics at the hadron colliders is to determine
the mass of invisible particles. This issue is particularly important for the
signals of dark matter, which becomes one of the prominent targets of future
collider experiments. In this paper, we show that the additional information
from the precise timing measurement, which will be available at the planned
high-liminosity run of the LHC (HL-LHC), will shade new light on the kinematics
study. As a concrete example, we focus on the signal of the pair produced
long-lived particles ($LLP_{1,2}$), respectively leaving displaced vertex with
visible ($V_{1,2}$) and invisible ($I_{1,2}$) final state, $pp \to LLP_1+LLP_2
\to (V_1+I_1)+(V_2+I_2)$. We explicitly show that this system is completely
solvable with timing information.
