将数字与日期时间/时间戳关联

Question

I have a dataframe df with a certain number of columns. One of them, ts , is timestamps:

1462147403122 1462147412990 1462147388224 1462147415651 1462147397069 1462147392497 ... 1463529545634 1463529558639 1463529556798 1463529558788 1463529564627 1463529557370 .

I have also at my disposal the corresponding datetime in the datetime column:

"2016-05-02 02:03:23 CEST" "2016-05-02 02:03:32 CEST" "2016-05-02 02:03:08 CEST" "2016-05-02 02:03:35 CEST" "2016-05-02 02:03:17 CEST" "2016-05-02 02:03:12 CEST" ... "2016-05-18 01:59:05 CEST" "2016-05-18 01:59:18 CEST" "2016-05-18 01:59:16 CEST" "2016-05-18 01:59:18 CEST" "2016-05-18 01:59:24 CEST" "2016-05-18 01:59:17 CEST"

As you can see my dataframe contains data accross several day. Let's say there are 3. I would like to add a column containing number 1, 2 or 3. 1 if the line belongs to the first day, 2 for the second day, etc...

Thank you very much in advance, Clement

Answer 1

One way to do this is to keep track of total days elapsed each time the date changes, as demonstrated below.

# Fake data
dat = data.frame(datetime = c(seq(as.POSIXct("2016-05-02 01:03:11"), 
                                  as.POSIXct("2016-05-05 01:03:11"), length.out=6), 
                              seq(as.POSIXct("2016-05-09 01:09:11"), 
                                  as.POSIXct("2016-05-16 02:03:11"), length.out=4)))
tz(dat$datetime) = "UTC"

Note, if your datetime column is not already in a datetime format, convert it to one using as.POSIXct .

Now, create a new column with the day number, counting the first day in the sequence as day 1.

dat$day = c(1, cumsum(as.numeric(diff(as.Date(dat$datetime, tz="UTC")))) + 1)

dat

  datetime day 1 2016-05-02 01:03:11 1 2 2016-05-02 15:27:11 1 3 2016-05-03 05:51:11 2 4 2016-05-03 20:15:11 2 5 2016-05-04 10:39:11 3 6 2016-05-05 01:03:11 4 7 2016-05-09 01:09:11 8 8 2016-05-11 09:27:11 10 9 2016-05-13 17:45:11 12 10 2016-05-16 02:03:11 15 

I specified the timezone in the code above to avoid getting tripped up by potential silent shifts between my local timezone and UTC. For example, note the silent shift from my default local time zone ("America/Los_Angeles") to UTC when converting a POSIXct datetime to a date:

# Fake data
datetime = seq(as.POSIXct("2016-05-02 01:03:11"), as.POSIXct("2016-05-05 01:03:11"), length.out=6)
tz(datetime)
[1] ""

date = as.Date(datetime)
tz(date)
[1] "UTC"

data.frame(datetime, date)

  datetime date 1 2016-05-02 01:03:11 2016-05-02 2 2016-05-02 15:27:11 2016-05-02 3 2016-05-03 05:51:11 2016-05-03 4 2016-05-03 20:15:11 2016-05-04 # Note day is different due to timezone shift 5 2016-05-04 10:39:11 2016-05-04 6 2016-05-05 01:03:11 2016-05-05