r - 在 R 中，为什么我会在 POSIXct 和 POSIXlt 之间获得一毫秒的差异？

Question

这个片段

options(digits.secs=3)
s<-"12:00:00.188"
fmt<-"%I:%M:%OS"
print(strptime(s,fmt))
print(as.POSIXct(strptime(s,fmt)))

给出这个文本输出:

[1] "2017-09-12 00:00:00.188 CEST"
[1] "2017-09-12 00:00:00.187 CEST"

虽然我希望上述结果是相同的。我错过了什么？

我的 session 信息:

print(sessionInfo())

给出:

R version 3.4.0 (2017-04-21)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 7 x64 (build 7601) Service Pack 1

Matrix products: default

locale:
[1] LC_COLLATE=Italian_Italy.1252  LC_CTYPE=Italian_Italy.1252    LC_MONETARY=Italian_Italy.1252 LC_NUMERIC=C                  
[5] LC_TIME=Italian_Italy.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

loaded via a namespace (and not attached):
[1] compiler_3.4.0 tools_3.4.0   

在 Linux 中的结果相同:

R version 3.3.3 (2017-03-06)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Debian GNU/Linux 9 (stretch)

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C               LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8     LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
 [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                  LC_ADDRESS=C               LC_TELEPHONE=C             LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

loaded via a namespace (and not attached):
[1] tools_3.3.3

编辑(在 Roland's comment 之后)

也许我错了，但在我看来 0.002 不能用浮点表示，仍然有 0.002 POSIXct 和 POSIXlt 之间没有区别:

options(digits.secs=3)
s<-"12:00:00.002"
fmt<-"%I:%M:%OS"
print(strptime(s,fmt))
print(as.POSIXct(strptime(s,fmt)))

给出:

[1] "2017-09-12 00:00:00.002 CEST"
[1] "2017-09-12 00:00:00.002 CEST"

Answer 1

您可以在 datetime 的文档中阅读有关此内容的信息:https://stat.ethz.ch/R-manual/R-devel/library/base/html/DateTimeClasses.html

特别是:

Class "POSIXct" represents the (signed) number of seconds since the beginning of 1970 (in the UTC time zone) as a numeric vector.

strptime和其他类(class) POSIXlt以不同的方式存储日期时间。

所以亚秒级精度存在问题:

Sub-second Accuracy

Classes "POSIXct" and "POSIXlt" are able to express fractions of a second. (Conversion of fractions between the two forms may not be exact, but will have better than microsecond accuracy.)

所以，你会看到 POSIXlt和 strptime准确打印这些:

strptime(s,fmt)
as.POSIXlt(strptime(s,fmt), format = "%Y-%m-%d %H:%M:%OS")

但是，因为 POSIXct进行计算以将日期表示为数字，由于浮点精度、闰秒等原因，它可能会不准确。