"Kilo" is 3 orders of magnitude larger than the base unit, while "milli" is 3 orders of magnitude smaller. That is, 1 second is [tex]10^{-3}[/tex] kiloseconds and [tex]10^3[/tex] milliseconds. This means 1 ks is [tex]10^6[/tex] ms, so
[tex]12.00\,\mathrm{ks}\cdot\dfrac{10^6\,\mathrm{ms}}{1\,\mathrm{ks}}=12.00\times10^6\,\mathrm{ms}[/tex]
