Many factors affect the accuracy of this calculator. Things like: Is the clock on the client-side synchronised with a time- or NTP-server? Are the regional settings properly configured on the client-side? But there are also other factors like delays caused by the internet or because of heavy load on your computer or the server.

Barring these factors, usually these times are accurate to within 0 to 30 seconds. Sometimes there can be a delay however, which is caused by the fact that the server cannot handle more than a small number of people going through the portal at once. When more than two people await the portal, the server will open the portal for some of them a minute or a few minutes later than the predicted time shown in this calculator. This means that the portals will open either at the predicted time or at most a few minutes later depending on how many people await the same portal.

See also the last paragraph of my Pod Age Portals Topic about portals appearing before the time indicated here.

According to the Uru Reallife website the first observed Negilahn portal was at 2007-03-03 18:33 UTC (03/03/2007 11:33 ki-time). This calculator places that portal at 03/03/2007 11:33:45 ki-time.