It's a long way to Alpha Centauri

using s=u t+1/2 a t^2, it can be found that 1.945 10^15 meters are traveled in the first two years. After that, the velocity is constant, and it takes 19.57 years to travel the remaining distance at 61740000 m/s

The distance traveled during the first two years is $d_1=\frac{1} {2} a t^2=0.49 \times (2 \times 3.15 \times 10^7)^2=1.94\times 10^{15}~\mbox{m}$ . The speed of the rocket after the first two years is $v=0.98 (2 \times 3.15 \times 10^7)$ . The remaining distance of $d_2=4 \times 10^{16}-1.94\times 10^{15}=3.81 \times 10^{16}~\mbox{m}$ is therefore traversed in $d_2/v=6.16 \times 10^8~\mbox{seconds}$ . The total time is this time (converted to years) plus two years or $21.6~\mbox{years}$ .