Zero, 628 CE.
Brahmagupta was a mathematician-astronomer of thirty when he wrote his Brahmasphutasiddhanta, and it's jam-packed with obviously brilliant things--theory of gravitation, position of the moon, sums of series, etc. The most brilliant aspect of all, however, is easy to overlook. By the time 628 came by, Indian mathematicians had already been using positional notations for some time. For those among you who don't do math, positional notation is the system we use; when we put down the number 6123, we're indicating how many different quantities-of-ten we add up to get the number we want by the position of each numeral--six thousands, one hundred, two tens, and three ones. If you needed to indicate that there were none of some quantity of tens, Indian mathematicians happily used a dot, meaning "nope" (or a cross, or, if you were Aryabhata, the syllable "kha", sort of) and found that it was much easier than doing calculations on an abacus in metered verse like before.
But ah ah! Not so fast. Having accepted that 6023 meant there was nothing in the hundreds place, mathematicians acknowledged that just 0 meant there was nothing at all. Which meant that presumably, you could use that 0 the way you used any other digit. But a number up to this point had meant the tally of how much of something there was. What the hell did it mean to calculate with a tally of nothing? What happened when you subtracted something from it? What happens when you multiply something by it? What are we even doing again?
This was a philosophical problem on a grand order, and Brahmagupta's solution was to think of it in terms of profit. When you were in debt, the debt was somehow less than having nothing at all, while a fortune, of course, was a surplus, so it's in those terms that Brahmagupta figured his rules for math. Take a debt away from nothing, and you're left with a fortune. Multiply a debt times a fortune and you're left with a debt. Anything multiplied by zero would give you a quantity of nothing and leave you with nothing again. The only thing he got wrong was division by zero, which still trips people up today (it's undefinable in real number arithmetic. Try that one out on an audience of seventh century mathematicians.)
Here is the awesome irony of this: when Brahmagupta was writing, positional notation was still apparently considered vulgar in serious mathematical work. He wrote his whole treatise in verse and used the names of mythological objects to stand in for numbers. The apotheosis of positional notation happened while people were still pretending it didn't exist.