Which R time/date class and package to use?

(I am moving this from the comments to the answer portion of stackoverflow at the request of the original poster.)

There is an article in R News 4/1 ("R Help Desk", p. 29) that specifically compares Date, POSIXct and chron. (The 1st two are in core of R and chron is a package.)

timeDate class (in the timeDate package) is based on POSIXct but has extra time zone/financial center support.

For regularly spaced series the the tis package supports many notions of dates.

The mondate package supports accounting dates.

The zoo time series package supports just about any date/time class and also has yearmon and yearqtr for ts compatibility.

The xts time series package works on top of zoo and handles the most common date/time classes by translating them to POSIXct and back again.

There is also information in the Time Series CRAN Task View.

Use POSIXct and lubridate.