See the FAQ: How to do a minimal reproducible example reprex for beginners for suggestions on posing questions more likely to attract quality answers. In R it always pays to begin with "what" before moving on to "how"?
The {lubridate} has good facilities for dealing with date times, so let's construct a toy data frame to see how you could make hourly time counts. The example below uses the analogous process of summarizing observations by day, since hourly data was easier to make up than by minute.
suppressPackageStartupMessages({
library(dplyr)
library(lubridate)
})
the_times = seq(
from=as.POSIXct("2021-1-1 0:00", tz="UTC"),
to=as.POSIXct("2021-1-11 15:00", tz="UTC"),
by="hour")
the_data <- sample(1:6,256,replace = TRUE)
DF <- data.frame(observed = the_data, time_observed = the_times)
head(DF)
#> observed time_observed
#> 1 5 2021-01-01 00:00:00
#> 2 3 2021-01-01 01:00:00
#> 3 4 2021-01-01 02:00:00
#> 4 2 2021-01-01 03:00:00
#> 5 3 2021-01-01 04:00:00
#> 6 4 2021-01-01 05:00:00
DF %>% group_by(Day = day(the_times)) %>% summarise(Count = sum(observed))
#> # A tibble: 11 × 2
#> Day Count
#> <int> <int>
#> 1 1 81
#> 2 2 82
#> 3 3 81
#> 4 4 81
#> 5 5 83
#> 6 6 77
#> 7 7 85
#> 8 8 70
#> 9 9 80
#> 10 10 88
#> 11 11 64