Converting Milliseconds to Minutes and Seconds?

I would suggest using TimeUnit. You can use it like this:

long minutes = TimeUnit.MILLISECONDS.toMinutes(millis);
long seconds = TimeUnit.MILLISECONDS.toSeconds(millis);

After converting millis to seconds (by dividing by 1000), you can use / 60 to get the minutes value, and % 60 (remainder) to get the "seconds in minute" value.

long millis = .....;  // obtained from StopWatch
long minutes = (millis / 1000)  / 60;
int seconds = (int)((millis / 1000) % 60);

tl;dr

Duration d = Duration.ofMillis( … ) ;
int minutes = d.toMinutesPart() ;
int seconds = d.toSecondsPart() ;

Java 9 and later

In Java 9 and later, create a Duration and call the to…Part methods. In this case: toMinutesPart and toSecondsPart.

Capture the start & stop of your stopwatch.

Instant start = Instant.now(); 
…
Instant stop = Instant.now();

Represent elapsed time in a Duration object.

Duration d = Duration.between( start , stop );

Interrogate for each part, the minutes and the seconds.

int minutes = d.toMinutesPart();
int seconds = d.toSecondsPart();

You might also want to see if your stopwatch ran expectedly long.

Boolean ranTooLong = ( d.toDaysPart() > 0 ) || ( d.toHoursPart() > 0 ) ;

Java 8

In Java 8, the Duration class lacks to…Part methods. You will need to do math as shown in the other Answers.

long entireDurationAsSeconds = d.getSeconds();

Or let Duration do the math.

long minutesPart = d.toMinutes(); 
long secondsPart = d.minusMinutes( minutesPart ).getSeconds() ;

See live code in IdeOne.com.

Interval: 2016-12-18T08:39:34.099Z/2016-12-18T08:41:49.099Z

d.toString(): PT2M15S

d.getSeconds(): 135

Elapsed: 2M 15S

Resolution

FYI, the resolution of now methods changed between Java 8 and Java 9. See this Question.

• Java 9 captures the moment with a resolution as fine as nanoseconds. Resolution depends on capability of your computer’s hardware. I see microseconds (six digits of decimal fraction) on MacBook Pro Retina with macOS Sierra.
• Java 8 captures the moment only up to milliseconds. The implementation of Clock is limited to a resolution of milliseconds. So you can store values in nanoseconds but only capture them in milliseconds.

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.

You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.

Where to obtain the java.time classes?

• Java SE 8, Java SE 9, Java SE 10, Java SE 11, and later - Part of the standard Java API with a bundled implementation.
  • Java 9 adds some minor features and fixes.
• Java SE 6 and Java SE 7
  • Most of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
• Android
  • Later versions of Android bundle implementations of the java.time classes.
  • For earlier Android (<26), the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use ThreeTenABP….