Refine condition and update help page

As discussed and co-developed with @hturner

Author: eddelbuettel

src/library/base/R/datetime.R

@@ -382,7 +382,7 @@ format.POSIXlt <- function(x, format = "", usetz = FALSE,
                            digits = getOption("digits.secs"), ...)
 {
     if(!inherits(x, "POSIXlt")) stop("wrong class")
-    if(any(f0 <- format == "")) {
+    if(any(f0 <- format == "" | grepl("%OS$", format))) {
         ## need list [ method here.
         times <- unlist(unclass(x)[1L:3L])[f0]
         np <- if(is.null(digits)) 0L else min(6L, digits)

src/library/base/man/strptime.Rd

@@ -39,7 +39,7 @@ strptime(x, format, tz = "")
     methods is
     \code{"\%Y-\%m-\%d \%H:\%M:\%S"} if any element has a time
     component which is not midnight, and \code{"\%Y-\%m-\%d"}
-    otherwise.  If \code{\link{options}("digits.secs")} is set, up to
+    otherwise.  If \code{\link{options}("digits.secs")} is set,
     the specified number of digits will be printed for seconds.}
   \item{\dots}{further arguments to be passed from or to other methods.}
   \item{usetz}{logical.  Should the time zone abbreviation be appended