>>What if the field is a year where -20,000,000 -> +20,000,000 are valid values (unless you are a creationist). What shall we you use then for 'unknown' . -1 and 999 will be a bit ambiguous....
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>Year 0 does not exist ...

True. But are you going to advocate that a value of zero should indicate 'not known' ?
TBH, to me, programming without null would be a bit like trying to be a mathematician before zero was invented :-}