Numbers, points and the colon

The one function

display.showNumberDecEx(1234, 0b01000000, true);
//                       ^     ^           ^
//                       |     |           leading zeros
//                       |     which points, counted from the left
//                       the number

Everything else in the library is a shorthand for this. showNumberDec(n) is this with no dots and no leading zeros.

Work the controls through the six numbers. Three of the combinations are not what you would expect, and all three cost somebody an evening at some point.

Zero loses its dots

With the number 0 and leading zeros off, the library takes a separate path — it blanks three digits, writes a single 0, and never applies the dots at all.

So a 24-hour clock written without leading zeros loses its colon for the whole of the minute after midnight. It is a genuine bug, it was fixed on the project's master branch in June 2020, and that fix has never been released: the version the Library Manager installs is still the one from 2018.

You do not need a patched library. Pass true for leading zeros and the special case never runs.

Long numbers lose their top

The library asks for four digits, takes them off the bottom of the number, and says nothing about what it dropped. 12345 shows as 2345 — not as an error, not as ----, just quietly wrong.

Four digits is 0 to 9999, and −999 to 9999 if you need the minus. Clamp in your own code:

if (value < 0 || value > 9999) display.setSegments(DASHES);
else display.showNumberDec(value);

The minus sign takes a digit

There is no separate minus; it is written into the first digit that would otherwise be blank. So −5 is -5, and −1234 has nowhere to put the sign and shows 1234 — a positive-looking number for a negative reading, which on a thermometer is the worst possible failure.

Together those two limits give a thermometer's real range on this display: −999 to 9999 in whole units, or −99.9 to 999.9 with a point.

And the points are independent

The number and the mask have nothing to do with each other. The same four bytes go out either way; the mask just sets bit 7 on the digits you name. Which is why 0b01000000 is a colon on one panel and a decimal point on the other, and why the same call can mean 12:34 or 12.34 depending on nothing but which display is soldered to the board.