Controlling plot axis for dates

I?d like to control the number of tick marks on the ?x? axis of a plot, 
when the variable there is dates.  I thought to use the xaxp parameter, 
but the documentation for par says ?It [i.e., xaxp] is only relevant to 
default numeric axis systems, and not for example to dates.?  My 
question, then, is how to control the number of ticks on an axis that 
represents dates?