It's about time you started thinking about setting up your own business.

[Freeguy]

Which of the following is better? I would use the first one.

It's about time you started thinking about setting up your own business.
It's about time you start thinking about setting up your own business.

 

[MikeNewYork]

They both work, but the first is more common.

 

[Rover_KE]

Only the first works for me.

 

[Tdol]

Maybe it's a BrE thing, but only the first works for me too.