adding a particular date to an SVG axis

#testing an answer I got on stack overflow to the following query I've created an axis with d3.svg.axis and a time scale and am happy with the ticks produced by the tick generator. However, I would like to ensure that a particular value is always marked. So for example the if the generator produces the following dates

2000-1-1, 2001-1-1, 2002-1-1, 2003-1-1

I might want to make the axis show

2000-1-1, 2000-7-21, 2001-1-1, 2002-1-1, 2003-1-1

How do i get an array of the ticks made by the tick generator so that I can add my value and pass it into the tickValues function?