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copied!<p>I've run into this problem, and so started looking for a solution. @Aaron's answer is good, but still breaks for large dates.</p> <p>Here is code that rounds the seconds properly, according to <code>format</code> or <code>option("digits.secs")</code>:</p> <pre><code>form <- function(x, format = "", tz= "", ...) { # From format.POSIXct if (!inherits(x, "POSIXct")) stop("wrong class") if (missing(tz) && !is.null(tzone <- attr(x, "tzone"))) tz <- tzone # Find the number of digits required based on the format string if (length(format) > 1) stop("length(format) > 1 not supported") m <- gregexpr("%OS[[:digit:]]?", format)[[1]] l <- attr(m, "match.length") if (l == 4) { d <- as.integer(substring(format, l+m-1, l+m-1)) } else { d <- unlist(options("digits.secs")) if (is.null(d)) { d <- 0 } } secs.since.origin <- unclass(x) # Seconds since origin secs <- round(secs.since.origin %% 60, d) # Seconds within the minute mins <- floor(secs.since.origin / 60) # Minutes since origin # Fix up overflow on seconds if (secs >= 60) { secs <- secs - 60 mins <- mins + 1 } # Represents the prior minute lt <- as.POSIXlt(60 * mins, tz=tz, origin=ISOdatetime(1970,1,1,0,0,0,tz="GMT")); lt$sec <- secs + 10^(-d-1) # Add in the seconds, plus a fudge factor. format.POSIXlt(as.POSIXlt(lt), format, ...) } </code></pre> <p>The fudge factor of 10^(-d-1) is from here: <a href="https://stackoverflow.com/questions/15383057/15391386">Accurately converting from character->POSIXct->character with sub millisecond datetimes</a> by Aaron.</p> <p>Some examples:</p> <pre><code>f <- "%Y-%m-%d %H:%M:%OS" f3 <- "%Y-%m-%d %H:%M:%OS3" f6 <- "%Y-%m-%d %H:%M:%OS6" </code></pre> <p>From a nearly identical question:</p> <pre><code>x <- as.POSIXct("2012-12-14 15:42:04.577895") > format(x, f6) [1] "2012-12-14 15:42:04.577894" > form(x, f6) [1] "2012-12-14 15:42:04.577895" > myformat.POSIXct(x, 6) [1] "2012-12-14 15:42:04.577895" </code></pre> <p>From above:</p> <pre><code>> format(t1) [1] "2011-10-11 07:49:36.2" > myformat.POSIXct(t1,1) [1] "2011-10-11 07:49:36.3" > form(t1) [1] "2011-10-11 07:49:36.3" > format(t2) [1] "2011-10-11 23:59:59.9" > myformat.POSIXct(t2,0) [1] "2011-10-12 00:00:00" > myformat.POSIXct(t2,1) [1] "2011-10-12 00:00:00.0" > form(t2) [1] "2011-10-12" > form(t2, f) [1] "2011-10-12 00:00:00.0" </code></pre> <p>The real fun comes in 2038 for some dates. I assume this is because we lose one more bit of precision in the mantissa. Note the value of the seconds field.</p> <pre><code>> t3 <- as.POSIXct('2038-12-14 15:42:04.577895') > format(t3) [1] "2038-12-14 15:42:05.5" > myformat.POSIXct(t3, 1) [1] "2038-12-14 15:42:05.6" > form(t3) [1] "2038-12-14 15:42:04.6" </code></pre> <p>This code seems to work for other edge cases that I've tried. The common thing between <code>format.POSIXct</code> and <code>myformat.POSIXct</code> in Aaron's answer is the conversion to from <code>POSIXct</code> to <code>POSIXlt</code> with the seconds field intact.</p> <p>This points to a bug in that conversion. I'm not using any data that isn't available to <code>as.POSIXlt()</code>.</p> <p><strong>Update</strong></p> <p>The bug is in <code>src/main/datetime.c:434</code> in the static function <code>localtime0</code>, but I am not sure yet of the correct fix:</p> <p>Lines 433-434:</p> <pre><code>day = (int) floor(d/86400.0); left = (int) (d - day * 86400.0 + 0.5); </code></pre> <p>The extra <code>0.5</code> for rounding the value is the culprit. Note that the subsecond value of <code>t3</code> above exceeds .5. <code>localtime0</code> deals with seconds only, and the subseconds are added in after <code>localtime0</code> returns.</p> <p><code>localtime0</code> returns correct results if the double presented is an integer value.</p>

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