How to count the 15-day visa exemption

Singaporean citizens are among those who are given free transit exemption if their stay in China is not more than 15 days. That's clear.
But there's some ambiguity as to how the days are computed.
If a Singaporean visitor arrives, without a visa, on 1 Jan say.
Counting 2Jan as day 1 would mean he can leave on 16 Jan.
Counting 1Jan as day 1, he has to leave on 15 Jan.
I have heard opinions in favour of both sides, and one saying it desn't matter
the Chinese Border officer will likely close an eye.
Can someone who is certain, provide some light?
Thanks.