How to fast convert different time formats in large data frames?

Question

I want to calculate length in different time dimensions but I have problems dealing with the two slightly different time formats in my data frame column.

The origin

Answer 1

or you can also use:

time<- c("2018-07-29T15:02:05Z",
         "2018-07-29T14:46:57Z",
         "2018-10-04T12:13:41.333Z",
         "2018-10-04T12:13:45.479Z")

length<-c(15.8,132.1,12.5,33.2)

df<-data.frame(time,length)
library(lubridate)

# df$time2<-as_datetime(df$time)
df$time2 <-parse_date_time(df$time, "ymd_HMS") 
df