test = tibble(time = c(as_datetime('2019-01-01 00:00:00'),
                       as_datetime('2019-01-01 01:00:00'),
                       as_datetime('2019-01-01 00:00:00'),
                       as_datetime('2019-01-01 01:00:00')),
              tz = c('EST','EST','Asia/Hong_Kong','Asia/Hong_Kong'))

test %>% group_by(tz) %>% mutate(localtime = with_tz(time, tz[1]))

  time                tz             localtime
  <dttm>              <chr>          <dttm>
1 2019-01-01 00:00:00 EST            2019-01-01 00:00:00
2 2019-01-01 01:00:00 EST            2019-01-01 01:00:00
3 2019-01-01 00:00:00 Asia/Hong_Kong 2019-01-01 00:00:00
4 2019-01-01 01:00:00 Asia/Hong_Kong 2019-01-01 01:00:00

test %>% group_by(tz) %>% mutate(localtime = with_tz(time, !!tz[1]))

   time                tz             localtime
  <dttm>              <chr>          <dttm>
1 2019-01-01 00:00:00 EST            2018-12-31 19:00:00
2 2019-01-01 01:00:00 EST            2018-12-31 20:00:00
3 2019-01-01 00:00:00 Asia/Hong_Kong 2018-12-31 19:00:00
4 2019-01-01 01:00:00 Asia/Hong_Kong 2018-12-31 20:00:00

test %>% rowwise %>% mutate(localtime = with_tz(time, tz))

 time                tz             localtime
<dttm>              <chr>          <dttm>
1 2019-01-01 00:00:00 EST            2019-01-01 08:00:00
2 2019-01-01 01:00:00 EST            2019-01-01 09:00:00
3 2019-01-01 00:00:00 Asia/Hong_Kong 2019-01-01 08:00:00
4 2019-01-01 01:00:00 Asia/Hong_Kong 2019-01-01 09:00:00