Thank you for your help. indexFormat(x) solved the problem nicely.
> head(a) 2011-03-04 09:30:00.0 22.10 2011-03-04 09:30:00.1 22.09 2011-03-04 09:30:00.2 22.10 2011-03-04 09:30:00.3 22.09 2011-03-04 09:30:00.4 22.10 2011-03-04 09:30:00.5 22.09 > indexFormat(a) <- "%H:%M:%OS3" > head(a) 09:30:00.000 22.10 09:30:00.100 22.09 09:30:00.200 22.10 09:30:00.300 22.09 09:30:00.400 22.10 09:30:00.500 22.09 -- View this message in context: http://r.789695.n4.nabble.com/xts-POSIXct-index-format-tp3336136p3337167.html Sent from the R help mailing list archive at Nabble.com. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.