Count the amount of observations in predetermined timestep - r

Question

I have a large dataset of over 75.000 observations. Of these observations I have a list of date and time combinations. I want to calculate the observation frequency in a predetermined timestep (15, 30 or 60 minutes). The study period is from 2014-10-21 00:00 to 2015-10-21 23:59.
The raw data is stored in a DF, but date (as POSIXlt) and time (as character) are in different columns, so I combine them back into one column to create a POSIXct timestamp.
receiver$date2 = as.POSIXct(paste(receiver$date, receiver$time), format="%Y-
%m-%d %H:%M:%S")
dateseq = receiver$date2
dateseq is now (only a small fragment using dput()):
dateseq = structure(c(1414140420, 1414140720, 1414140960, 1414141080, 1414143540, 1414144980, 1414145940, 1414147380, 1414147440, 1414148100, 1414148280, 1414152720, 1414153740, 1414154520, 1414154580, 1414158540, 1414159380, 1414159680, 1414164240, 1414164300, 1414164840, 1414164900, 1414165500, 1414166100, 1414166220, 1414166460, 1414166520, 1414166820, 1414166880, 1414166940, 1414167300, 1414167360, 1414167480, 1414167780, 1414168380, 1414168440, 1414168800, 1414168860, 1414202040, 1414202220, 1414202280, 1414202700, 1414202820, 1414202880, 1414203660, 1414203960, 1414215180, 1414215300, 1414215900, 1414216560, 1414216860, 1414217220, 1414217280, 1414217460, 1414217580, 1414217700, 1414217820, 1414217880, 1414218240, 1414218720, 1414219380, 1414219800, 1414219920, 1414219980, 1414220160, 1414220280, 1414220820, 1414220880, 1414221000, 1414221960, 1414222080, 1414222200, 1414222320, 1414222500, 1414222560, 1414222860, 1414223640, 1414224780, 1414225800, 1414225920, 1414225980, 1414226040, 1414226100, 1414226220, 1414227240, 1414227420, 1414227600, 1414230300, 1414230540, 1414230840, 1414231140, 1414231320, 1414231440, 1414231560, 1414231800, 1414231860, 1414232040, 1414232160, 1414232400, 1414232520, 1414232640, 1414232700, 1414232760, 1414232880, 1414232940, 1414233060, 1414233180, 1414233240, 1414233300, 1414233420, 1414233480, 1414233660, 1414233720, 1414233780, 1414233840, 1414233960, 1414234080, 1414234320, 1414234440, 1414234560, 1414234620, 1414234740, 1414234860, 1414234980, 1414235040, 1414235280, 1414236240, 1414236300, 1414236420, 1414236540, 1414236840, 1414236900, 1414236960, 1414237020, 1414237260, 1414237560, 1414237860, 1414238280, 1414238400, 1414238460, 1414238580, 1414238640, 1414239180, 1414239300, 1414239360, 1414239480, 1414239540, 1414240440, 1414240860, 1414240920, 1414240980, 1414241040, 1414242000, 1414242180, 1414242480, 1414242540, 1414242660, 1414242720, 1414242840, 1414242900, 1414243800, 1414243920, 1414244280, 1414244460, 1414245240, 1414245600, 1414245660, 1414246080, 1414246500, 1414246680, 1414246740, 1414246920, 1414247340, 1414248180, 1414249320, 1414249560, 1414249860, 1414250340, 1414250520, 1414250640, 1414250760, 1414250880, 1414250940, 1414251060, 1414251240, 1414251900, 1414252020, 1414252080, 1414252200, 1414252260, 1414252380, 1414252440, 1414252440, 1414252500, 1414252560, 1414252680, 1414252980, 1414253160, 1414253460, 1414253580), class = c("POSIXct", "POSIXt"), tzone = "")
Then I want to have a timeseq that runs for the whole period (so also the days that don't have any observations) divided by the predetermined timestep.
timestep = 1800 # 1800 sec = 30 min
start = "2014-10-21 00:00"
end = "2015-10-21 23:59"
receiver = R125926
timeseq = seq(from = as.POSIXct(start), to = as.POSIXct(end), by = timestep)
Now I want to 'fill' a new dataframe with the timeseq in one column and the count data of how many observations (from dateseq) occurred in that time period.
EDIT
After some searching on the forum and adjusting some code, I came to one very simple method that brings me very close to what I want my results to look like:
det_interval = data.frame(table(cut(dateseq, breaks = "30 min")))
There's only two adjustments that I don't know how to do. Now it begins at the first record (e.g. when my first record is on 05.17 the interval it will use will be x.17 - x.47 (30min)), not at the start that I want (see the timeseq created above). So how can I make sure that this starts and ends at a predetermined date/time?