The calculation method of System.currentTimeMillis() and the conversion of time units.

The System.currentTimeMillis() method is used to obtain the current system time in milliseconds, which is the number of milliseconds that have passed since midnight on January 1, 1970 GMT.

Unit conversion of time can be achieved by dividing or multiplying by the corresponding factor.

1 second is equal to 1000 milliseconds
1 minute is equal to 60 seconds, which is equal to 60 * 1000 milliseconds
1 hour is equal to 60 minutes, which is equal to 60 * 60 * 1000 milliseconds
1 day is equal to 24 hours, which is equal to 24 * 60 * 60 * 1000 milliseconds
1 week is equal to 7 days, which is equal to 7 * 24 * 60 * 60 * 1000 milliseconds

For example, you can convert the return value of System.currentTimeMillis() to seconds by dividing by 1000.

We are first getting the current time in milliseconds using the System class, and then converting it into seconds by dividing by 1000.

To convert seconds to the return value of System.currentTimeMillis(), you can multiply by 1000.

Convert the current time in seconds to milliseconds by multiplying it by 1000.

It is important to note that System.currentTimeMillis() returns the number of milliseconds that have passed since January 1, 1970, 00:00:00 GMT, so when converting units, the effects of the time zone need to be considered.