Create new section and take time difference if time exceeds a certain amount (R, Dplyr)

I have a dataset, df, that has 10,000 rows:

DateA

9/9/2019 7:52:16 PM
9/9/2019 7:52:16 PM
9/9/2019 7:52:17 PM
9/9/2019 7:52:18 PM
9/9/2019 7:52:18 PM
9/9/2019 7:52:19 PM
9/10/2019 1:02:23 AM
9/10/2019 1:02:25 AM
9/10/2019 1:02:26 AM
9/10/2019 1:02:27 AM
9/10/2019 1:02:27 AM
9/10/2019 1:02:29 AM
9/10/2019 1:02:29 AM
9/10/2019 1:03:29 AM    
9/10/2019 1:03:29 AM    
9/10/2019 1:03:31 AM    
9/10/2019 1:03:32 AM    
9/10/2019 4:18:48 AM    
9/10/2019 4:18:50 AM    
9/10/2019 4:18:51 AM    

I would like this output:

Group   Duration

 a       3 sec
 b       6 sec
 c       3 sec
 d       3 sec

I would like the thresh to be set to 1 min or 60 sec. If a lapse of more than 60 seconds is detected, a new group will be made, along with its duration.

dput:

 structure(list(DateA = structure(c(12L, 12L, 13L, 14L, 14L,   15L, 
 1L, 2L, 3L, 4L, 4L, 5L, 5L, 6L, 6L, 7L, 8L, 9L, 10L, 11L),      .Label = c("9/10/2019 1:02:23 AM", 
"9/10/2019 1:02:25 AM", "9/10/2019 1:02:26 AM", "9/10/2019   1:02:27 AM", 
"9/10/2019 1:02:29 AM", "9/10/2019 1:03:29 AM", "9/10/2019 1:03:31 AM", 
"9/10/2019 1:03:32 AM", "9/10/2019 4:18:48 AM", "9/10/2019 4:18:50 AM", 
"9/10/2019 4:18:51 AM", "9/9/2019 7:52:16 PM", "9/9/2019 7:52:17 PM", 
"9/9/2019 7:52:18 PM", "9/9/2019 7:52:19 PM"), class =  "factor")), class =   "data.frame", row.names = c(NA, 
 -20L))

I have tried:

 thresh1 <-60

 library(data.table)
 setDT(df)[, DateA := as.ITime(as.character(DateA))][, 
  .(Duration = difftime(max(as.POSIXct(DateA)),         min(as.POSIXct(DateA)), 
   unit = 'sec')),.(group = letters[cumsum(c(TRUE, diff(DateA) >     thresh1))])]

However, I am doing something wrong because I am only getting the output of 1 row.

  group  Duration

   a       0

Not sure what I am doing wrong? Any suggestion is appreciated.