By default the timestamp is based on GMT, for example, “2017-01-01 00:30:00 GMT”. I want to change it to local time like EST. However, the code below seems not working.

df$TIMESTAMPE_EST = as.POSIXct(df$TIMESTAMP, format = "%Y-%m-%d %H:%M:%S", tz = "EST")
head(df.PPFD$TIMESTAMPE_EST)
[1] "2017-01-01 00:30:00 GMT" "2017-01-01 01:00:00 GMT" "2017-01-01 01:30:00 GMT" "2017-01-01 02:00:00 GMT" "2017-01-01 02:30:00 GMT" "2017-01-01 03:00:00 GMT"