R Plot Specify number of time tickmarks - time/date equivalent to pretty

I was wondering how I could plot more tick marks when plotting time on the x-axis.

Basically, a time equivalent to pretty. Pretty obviously doesn't work so well with times, as it uses factors of 1,2,5 and 10. For time one probably wants e.g. hours, half hours, ...

plot(as.POSIXct(x,origin="1960-01-01"),y,type="l",xlab="Time")

gives really too few and widely spaced tickmarks.

zoox<-zoo(y,as.POSIXct(stats$Time,origin="1960-01-01"))
plot(zoox)

gives the same.

Thanks

EDIT:

Just to clarify (so far answers don't address my issue): What I am looking for is a function like pretty for dates, e.g. a function, that takes a start date, an end date, a number of ticks, and outputs the location of the ticks. That is, I am well aware it is possible to plot hours, to plot minutes, and what else, but pretty automates the tick distance for numbers, and a resulting function for dates should decide by itself whether to use days, hours, minutes, second, milliseconds, microseconds, 30 minutes, 500 micros, 5 seconds, etc. intervals. That is what pretty does for numbers, anyway.

EDIT2:

This is the function I currently use to decide the format for the time axis (note that this doesn't work for dates):

mydiff <- end-start
if(mydiff>1800) {
    axis.POSIXct(1,xrange,format="%H:%M")
} else if(mydiff>30) {
    axis.POSIXct(1,xrange,format="%H:%M:%S")
} else if(mydiff>0.5) {
    axis.POSIXct(1,xrange,format="%H:%M:%OS3")
} else
    axis.POSIXct(1,xrange,format="%H:%M:%OS6")
}

I don't have a function that increase tick marks, so I use the default number of tick marks