What is my Day Master?
The Day Master (日主) is the top character of your day pillar. It is the reference point the rest of the chart is read against — everything else is described in relation to it.
Example — born 15 June 1990, 12:00, Hong Kong
Day pillar 辛亥 — so the Day Master is 辛, the top character.
Two reasons sites disagree about it
The day boundary
A birth between 23:00 and midnight sits in the zi hour, and schools split on whether the day pillar has already rolled over. That changes the Day Master itself, not just a detail. This engine keeps the current day and shows you the alternative — the day-boundary cases record both results for each birth near 23:00.
Which clock the boundary is read on
If the day starts at 23:00, then 23:00 by which clock? A birth far from its time zone's meridian can be on either side depending on whether true solar time is applied. At Hong Kong's longitude the combined correction runs about −24 minutes — enough that a clock reading of 23:00 is still 22:36 true solar time, comfortably before the boundary. Most calculators do not say which clock they use, so their Day Masters drift silently for exactly the births nearest the line.
Once you have it
The Day Master is one of ten possible stems. Reading a chart means describing the other seven characters' relationship to it — which is why getting the day pillar wrong invalidates everything downstream, and why the free calculator prints the full correction chain it used rather than just the result.
Find your Day Master — the chart shows which side of the boundary you fall on and why. New to the chart itself? Start with what the four pillars are.