Which Calendar Is the Best?
There is only 1 day in history when Guru Gobind Singh Ji was born. But there are many ways to represent that historic day. Here are just a few:
23 Poh, 1723
January 1st, 1667
Rajab 6, 1077
5th of Tevet, 5427
Each of the dates listed above is from a different calendar which originated in different parts of the world and they're all perfectly good ways of referring to the same historic day. But what happens when different calendars point to different anniversaries for the same historical day? For example, when one calendar says a Gurpurab is Tuesday and another says Saturday, how do we know which one is correct?
The correct date to celebrate is the one you believe is most accurate. The fact is no calendar in the world is perfect because the length of a year isn't perfect. A year is actually about 365.25 days long. That extra 0.25 day means that every 3 years all our dates are about 18 hours off, almost a full day behind the correct date. Leap years correct that problem but the cycle continues again. In addition, the Earth's tides make slight changes to our planet's rotation and orientation. With the passage of time, these changes make days slightly longer resulting in changes to how we calculate the length of a year. On top of these natural discrepancies, there is the issue of time zones. Those of us living in the West are one day behind Asia due to the location of the international date line. So no matter which date you choose to celebrate a Sikh holiday, remember that it's just an approximation.
Although no calendar is perfect, some are better than others. But the difference is usually just a matter of a few days. So pick a date that you believe is correct and go with it. If you must know the exact day and hour that one of the Gurus was born, throw away all the current calendars because none of them are accurate enough. Even without calendars there is a seasonal rhythm and order that holidays should be celebrated in which will always remain the same. So if you live in the Northern Hemisphere, Vaisakhi should always occur around the beginning of springtime and Guru Gobind Singh Ji's Parkash Utsav (birth anniversary) should always occur around the shortest day of the year.
No matter which date you choose to celebrate, keep in mind that measuring time is a thoroughly human pursuit, God is Akal (timeless).