You might have already heard that one of the magic numbers in Chinese criminal procedure is 37.
Individuals placed under criminal detention by investigators can be held in custody for a maximum of 37 days without any intervention by any other authority. That is, criminal detention can last for up to 30 days, at which point the investigating body must (a) apply to the procuratorate (prosecutor's office) for permission to carry out formal arrest, (b) release the suspect on guarantee pending further investigation or into residential surveillance, or (c) release the suspect outright. In case of (a), the procuratorate then has seven days in which to make a decision, during which time the suspect generally remains in custody—so, a total of 37 days.
Here, I'll only mention in passing that criminal detention is, in theory at least, not intended to be used nearly as routinely as it is and that the degree to which detentions are regularly stretched to their 30-day limit also goes well beyond the rather limited set of circumstances for which such detention is provided by law. (For more on this subject, see this.)
My real interest, though, is the question of how to calculate the relevant deadlines with respect to this magic number. It's not nearly as simple as you might think.
First of all, one must be aware that criminal detention may be preceded by up to 24 hours of questioning under the procedure of criminal summons. So, for example, if police come to my home on 1 May with a warrant for me to appear for questioning, they can question me at the police station for up to 24 hours before issuing an official detention notice. Only after this notice is issued am I considered to be under criminal detention.
The second complicating factor is that Article 103 of the Criminal Procedure Law states, in part: "Time periods are calculated in hours, days, and months. The initial hour or day does not count toward the time period."
What does this mean in practice? Let's say my criminal detention notice was issued at 8 p.m. on 1 May. For the purposes of calculating my detention, the clock starts ticking at midnight on 2 May, meaning that 30 days would expire at 11:59:59 p.m. on 31 May. Note that this would be true whether I was placed under detention at 12:01 a.m. or 11:59 p.m on 1 May. In any of these cases, for the purposes of counting the days, you would start from midnight on 2 May.
So, in this hypothetical situation, if the police wanted to arrest me, they would have to send their request to the procuratorate on 31 May at the latest. The subsequent seven days would start from midnight on 1 June, meaning that the procuratorate would have to issue a decision by the end of the day on 7 June.
It's worth pointing out that, other than what I've cited above, the law is not too specific about when to start counting. What I've provided here is the most common scenario, likely to apply in the majority of instances. But, as they say, your mileage may vary.