问题
I want to round a POSIXct down to the day, relative to a specific timezone.
If I try
round(as.POSIXct("2013-03-05 23:00:00 EST"), "day")
It returns
2013-03-06
Which makes sense, in that when it's 23:00:00 EST on 2013-03-05 in EST5EDT, it's already 2013-03-06 in UTC. Logically, what I want to do is:
round(as.POSIXct("2013-03-05 23:00:00 EST"), "day", tz="EST5EDT")
That is, "round this date and time to the nearest day, relative to the EST5EDT time zone". Unfortunately, round doesn't take a time zone parameter.
回答1:
round will round to the next day once it's past midday, which is why I think you are seeing 2013-03-06. I also have to explicitly set the tz argument in the call to as.POSIXct
Observe:
round( as.POSIXct("2013-03-05 11:00:00" , tz = "EST" ), "day" )
[1] "2013-03-05 EST"
And then once it passes noon:
round( as.POSIXct("2013-03-05 12:00:00" , tz = "EST" ), "day" )
[1] "2013-03-06 EST"
A call to format extracts the day as a character string without the tz argument. So you can get your original result without the timezone
format( round( as.POSIXct("2013-03-05 12:00:00" , tz = "EST" ), "day" ) )
[1] "2013-03-06"
If you want to round any time on that day to that day perhaps what you want instead is trunc?
format(trunc( as.POSIXct("2013-03-05 12:00:00" , tz = "EST" ), "day" ))
[1] "2013-03-05"
来源：https://stackoverflow.com/questions/16041093/round-a-posix-date-and-time-posixct-to-a-date-relative-to-a-timezone