I'm using cacti, and I accidentally set my clock set ahead. Cacti updated, storing new data in the rra's - and now that I've fixed my clock, I can't get cacti to update again. What do I do?
You have to erase these invalid future entries. Use rrddump to export your rra to an XML file, then edit the XML file. Set all the future time to NaN, and set the lastupdate to 0. Restore the RRD files using rrdrestore.
Note, however, that depending on how far ahead your clock was set, you may have lost all your data! For a 5 minute average over a day, a clock set ahead 5 days would have wiped it all out! Same story if you accidently set your clock ahead to 2022 - even a 5 year rra will be completely wiped out. This is why it's important to backup your rra's - clock sync is essential.