The Lorentz factor, due to relativity, also known as the Time Dilation Factor, is defined thus:

$\gamma =\frac{1}{\sqrt{1-\frac{v^2}{c^2}}}$

Where,

v Is the object’s velocity

c Is the speed of light

$\frac{v^2}{c^2}={\left(\frac{v}{c}\right)}^2$

$\frac{v}{c}=0.9\to$

$\gamma =\frac{1}{0.43588989...}$

Dirk