[R] correct my method of estimating mean of two POSIXlt data frames

[gotrout@gmail.com]

> If a.lt and b.lt are the two vectors of POSIXlt dates then try
> converting each to POSIXct and unclassing to make each numeric.
> Take the mean of the two numeric vectors and convert them back to

I see.  I'm a little confused with the use of class/unclass versus
as.XX.  For example, instead of using unclass, why wouldn't I use
as.numeric?  Could someone explain the difference?
thanks,
Mike