Problem with Time series data entry

I have some historical time series data.  The data was collected approx. every 15 mins, I say approx. because the times are not exactly 15 mins to the second apart.  In some cases there could be 10-15 seconds of delay  between each of the 15 min collections.  In some cases if a collection cycle was skipped a time entry less then 15 mins will occur.   For example, data collected at 10:00:01, 10:15:07, 10:30:03, 10:45:11, 10:48:13, 10:51:12, 11:00:32.

I have created the following:
rrdtool create test.rrd --start 1179721292 --step 900 DS:value:GAUGE:1830:-5:5000 RRA:LAST:0.5:1:672 RRA:AVERAGE:0.5:4:2160

I expected this to allow me to keep the actual measured value and the time the value was collected for 7 days and then the hourly average for the previous 90 days.

I update test.rrd with the following:

rrdtool update test.rrd 1179722192:0.359
rrdtool update test.rrd 1179723089:0.343
rrdtool update test.rrd 1179724001:0.329
rrdtool update test.rrd 1179724892:0.359
rrdtool update test.rrd 1179725797:0.36
rrdtool update test.rrd 1179726695:0.375
rrdtool update test.rrd 1179727604:0.36
rrdtool update test.rrd 1179728493:0.359
rrdtool update test.rrd 1179729394:0.375
rrdtool update test.rrd 1179730293:0.344

When I perform a rrdtool fetch the first data entry I get has a timestamp of 1179721800 this is not the date/time I passed in with the update.

Is it not possible to collect/store and then graph time series data using the exact time the measurement was taken.

Any help/guidance would be appreciated.

Chris G.