I'm setting up a rrd database to store sensor data for 3 days in 12hr intervalls (43200s) = 6 row in RRA.
rrdtool create test.rrd --step 43200 --start 1562429286 DS:temp:GAUGE:86400:U:U RRA:AVERAGE:0:1:6
The databases starting time is 1562429286 (06.07.2019 - 18:08:06).
When I dump the database:
rrdtool dump test.rrd
it says (output trimmed for clarity):
2019-07-04 02:00:00 CEST / 1562198400 NaN
2019-07-04 14:00:00 CEST / 1562241600 NaN
2019-07-05 02:00:00 CEST / 1562284800 NaN
2019-07-05 14:00:00 CEST / 1562328000 NaN
2019-07-06 02:00:00 CEST / 1562371200 NaN
2019-07-06 14:00:00 CEST / 1562414400 NaN
I expected rrdtool to give the next nearest timestamp ( 6.7.19 18:00 ) as the last entry ("starting point") instead. So why is it at 14:00 ? At first this explanation (How to create a rrd file with a specific time?) made perfect sense for the small intervall of 5m to me. But in my case I cannot get behind the logic if the intervall is bigger (12h)
This is because the RRA buckets are always normalised to be aligned to the GMT (UCT) timezone. It is not visible if you are using a cdp (consolodated data point) width of an hour or less; but in your case, your cdp are 12 hours in width. Your timezone means that these are offset by 2 hours from UCT zero resulting in apparent boundaries of 02 and 14 local time (if you were in London then you'd be seeing 0 and 12 as expected).
This effect is much more noticeable when you are using 1-day rollups and are located in somewhere like New Zealand, when you'll see the CDP boundary appearing at noon rather than at midnight.
It is not currently possible to specify a different timezone to use as a base for the RRA buckets (this would make the data nonportable) though I believe it has been on the RRDTool feature request list for a number of years.