Date & Time Master Calculator

This is an all-in-one date and time calculator with five modes selected from a dropdown, and by default it opens on Calculate Age, which is what this explainer and its claims describe. In that mode it builds two dates from the Year/Month/Day fields you provide -- one for the earlier date, one for the later -- and runs a calendar-borrow age calculation, the same style as a standalone age calculator: it subtracts month and day components directly, then borrows a month, adding that specific month's real length, if the day difference goes negative, and borrows a year if the month difference then goes negative too (lines 226-243). Total Days is the plain difference between the two dates in whole days (line 54), and Total Weeks is that figure divided by seven (line 56); Total Months, though, is not built from Total Days at all -- it's the calendar-borrow years converted to months plus the leftover months (line 57), which is more precise than a fixed days-per-month average would be, since it accounts for the real, varying length of every month in between. Month (Start/Birth) usually has the largest effect on Days Until Birthday of this mode's inputs, because it selects which month the next-birthday date rolls to (lines 245-253) -- picking a different birth month can swing the next birthday across the whole calendar year, while Day (Start/Birth) usually only shifts that date within a single month.

That ranking flips right at the rollover point, though: whenever the reference date sits close enough to the upcoming birthday's month and day that a small change in Day (Start/Birth) pushes the calculated birthday across this year versus next year, that single day's change can swing Days Until Birthday by nearly a full year -- a bigger jump than an ordinary change in Month (Start/Birth) produces away from that boundary. Unix Timestamp, meanwhile, is hidden in this mode, since the Calculate Age branch never reads it -- it only matters, and only becomes visible, when the mode dropdown is switched to Unix Timestamp to Date, the one branch that actually reads the field (Date to Unix Timestamp computes its own timestamp from the Year/Month/Day fields instead, so it doesn't read this field either). Next Birthday looks at whether this year's birthday, built from the earlier date's month and day, has already passed the later date; if it has, it rolls the target forward to next year before measuring the gap in Days Until Birthday.