The sample data you have posted has a %d/%m/%Y %H:%M:%S format, not %Y/%m/%d %H:%M:%S, If I change that everything works as expected.
# Sample data in a copy/paste friendly format, replace this with your own data frame
calories <- data.frame(
stringsAsFactors = FALSE,
ID = c(2022484408,2022484408,
2022484408,2022484408,2022484408,2022484408),
TIME = c("4/12/2016 7:21:00 AM",
"4/12/2016 7:21:05 AM","4/12/2016 7:21:10 AM",
"4/12/2016 7:21:20 AM","4/12/2016 7:21:25 AM","4/12/2016 7:22:05 AM"),
VALUE = c(97, 102, 105, 103, 101, 95)
)
# Relevant code
calories$TIME <- as.POSIXct(calories$TIME, format="%d/%m/%Y %H:%M:%S", tz="UTC")
calories
#> ID TIME VALUE
#> 1 2022484408 2016-12-04 07:21:00 97
#> 2 2022484408 2016-12-04 07:21:05 102
#> 3 2022484408 2016-12-04 07:21:10 105
#> 4 2022484408 2016-12-04 07:21:20 103
#> 5 2022484408 2016-12-04 07:21:25 101
#> 6 2022484408 2016-12-04 07:22:05 95
Created on 2022-01-18 by the reprex package (v2.0.1)