library(tidyverse)
library(lubridate)
(df1 <- tibble(a="a",
b="2020-12-21 09:00:00 -0600/2020-12-21 09:30:00 -0600"))
(df2 <- df1 %>% mutate(splt = str_split(value1,"/",simplify = FALSE)))
# i ignore the timezone as it doesnt seem relevant to you (unless you have mixed timezone data ??)
(df3 <- df2 %>% mutate(pdt1 = parse_datetime(str_sub(splt[[1]][1],start = 1,end = -7)),
pdt2 = parse_datetime(str_sub(splt[[1]][2],start = 1,end = -7))))
(df4 <- df3 %>% mutate(intrvl = as.duration(lubridate::interval(start=pdt1,end = pdt2))))
select(df4,
intrvl)