These portal times are regularly verified within MOUL. The portal times from the
other Shards usually come very close to these times.
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
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 few paragraphs of my
Pod Age Portals Topic
about portals appearing before the time indicated here.