The speed of light (c), gravitational constant (G) and Planck's constant (h) are taken as the fundamental units  in a system. The dimension of time in this system is:

$\text{ Time }\propto c^x{G}^y{h}^z\Rightarrow T=k{c}^x{G}^y{h}^z$

Putting the dimensions in the above relation

$\begin{array}{l}\Rightarrow \left[M^0{L}^0{T}^1\right]={\left[L{T}^{-1}\right]}^x{\left[M^{-1}{L}^3{T}^{-2}\right]}^y{\left[M{L}^2{T}^{-1}\right]}^z\\ \Rightarrow \left[M^0{L}^0{T}^1\right]=\left[M^{-y+z}{L}^{x+3y+2z}{T}^{-x-2y-z}\right]\end{array}$

Comparing the powers of M, L and T

$\begin{array}{l}-y+z=0 ...........\left(i\right)\\ x+3y+2z=0 ...........\left(ii\right)\\ -x-2y-z=1 ..........\left(iii\right)\end{array}$

On solving equations (i) and (ii) and (iii)

$x=\frac{-5}{2},y=z=\frac{1}{2}$

Hence dimension of time are $\left[G^{1/2}{h}^{1/2}{c}^{-5/2}\right]$