Using R, how can I correctly subtract a second from a POSIXct variable using dplyr's lead and lag functions?

I have 12 dates and times in a table in POSIXct format:

                Time
1  2017-03-11 01:10:09
2  2017-03-11 03:07:58
3  2017-03-12 19:16:47
4  2017-03-13 09:52:04
5  2017-03-17 20:36:35
6  2017-03-18 03:10:54
7  2017-03-18 07:29:31
8  2017-03-18 10:13:37
9  2017-03-20 10:19:31
10 2017-03-20 12:11:39
11 2017-03-20 12:11:39
12 2017-03-20 14:16:12

If an entry matches the following entry, I want to remove one second from the time. For example, row 10 should appear as "2017-03-20 12:11:38".

I'm trying to do something like the following but that works:

df %>% mutate(Time = ifelse(Time == lead(Time), Time-1, Time))

Solution

Your approach is correct however, base::ifelse strips off the attribute making POSIXct column as numeric. To avoid that since you are using dplyr you can use dplyr::if_else which preserves the class.

Also note that lead will generate NA for last value in Time so the comparison Time == lead(Time) would generate NA. We could set missing argument as Time in if_else.

library(dplyr)
df %>% mutate(Time = if_else(Time == lead(Time), Time-1, Time, missing = Time))

#                  Time
#1  2017-03-11 01:10:09
#2  2017-03-11 03:07:58
#3  2017-03-12 19:16:47
#4  2017-03-13 09:52:04
#5  2017-03-17 20:36:35
#6  2017-03-18 03:10:54
#7  2017-03-18 07:29:31
#8  2017-03-18 10:13:37
#9  2017-03-20 10:19:31
#10 2017-03-20 12:11:38
#11 2017-03-20 12:11:39
#12 2017-03-20 14:16:12