Yes, smartphones are Android / iPhone, and basically no time adjustment is required. If you insert the SIM card of the contracted communication carrier and connect it to the mobile line, it will keep ticking the time fairly accurately. All you have to do is carry it around as a smartphone, and the so-called "clock will not go wrong".
This is because most smartphones currently on the market support "NITZ (Network Identity and Time Zone)". NITZ is a standard that provides information such as the current time and time zone via a base station, and is provided by a communication carrier. If you are connected to the line of the communication carrier that provides NITZ, the time will be adjusted regularly, so maintenance is not required. In the past, there was a difference in whether or not it was supported depending on the communication carrier, but as of 2019, all three major carriers in Japan provide it, so all lines including MVNO are compatible with NITZ.
To set the time by NITZ, go to "Settings"-> "System"-> "Date and Time" on Android OS, "Automatic Date and Time" switch, and "Settings"-> "General"-> "Date and Time" on iOS. It is enabled by turning on a certain "automatic setting" switch. If the switch is turned off, the time information provided by NITZ will not be referenced, and the current time will shift after a while.
The NITZ standard stipulates that the time error between Coordinated Universal Time (UTC) and base station equipment should be kept to 1.5 microseconds or less. For the general public, the smallest unit of time handled on a daily basis is "seconds", which is more accurate than enough, but it is not as accurate as an atomic clock in femtoseconds (10 minus 15 seconds).
