We Are Not Experts When Hatzot Is
We are approaching Rosh Hodesh Sivan, which means that the time of Minha Gedola of the sixth millennium is only a bit more than 2 months away - on ר"ח אב תשע"א - Aug. 1st, 2011.
For reference, see:
I'm no Harold Camping - lehibadel - but this date is an auspicious time.
Many were disappointed when Mashiah didn't arrive in 5751 - what we considered Hatzot of the sixth millennium.
According to the Magen Avraham in Orah Hayim 233:1 (quoted by Sha'arei Teshuva and Sha'ar Hatziyun here), we really should start praying Minha at Hatzot. However, we're not experts on the exact time of Hatzot Hayom - midday. Therefore, we push it off a half an hour so that we're certain that the sun is in the West.
Similarly, our calendar may be off enough for us to mistake when Hatzot really is. Therefore, Hashem may have pushed off the date of Mashiah to what we consider Minha Gedola - 5771 - but may really be Hatzot.
A similar idea can be found during the time of the Exodus from Egypt, when no human knew the exact time when midnight was. Moshe Rabbeinu announced כחצות הלילה - around midnight - knowing full well that Hashem had told him that the plague of the firstborn would be done exactly at midnight. He did so to make sure the Egyptian stargazers won't think that Hashem is lying if it's off according to their calculations.
We are like the Egyptian stargazers. We don't know when the exact time will occur. We don't even know if Minha Gedola is the right time. However, we know that it's auspicious. And we hope.