cosmic time vs. redshift.

The relation is 

t=28/[1+(1+z)^2] Gyr

http://arxiv.org/pdf/gr-qc/0506079v2

If I set the time to be 1 Gyr, then I will get the redshift to be 4.2, which means that the redshift should be higher than 4.2 in order to get the cosmic time less than 1 billion year.