Java how to add time and date - datetime

Question

Using JDK 1.8
I have Time in ms for a day (not since 1970) and I have a Date how do I add the two and create a datetime.
Thanks
Tried #Kikos soln does not produce correct result:
Somehow my orig time 930 hrs in this case changes to 9:40, the orig date itself should not have any time gets time (??) - so the addition math fails.
String testdate = "2015/10/25";
Date date = new SimpleDateFormat("yyyy/mm/dd").parse(testdate);
String timehrs= "930";
long ltime = Long.parseLong("930");
long hoursAsSeconds = (ltime / 100) * 60 * 60;
long minsAsSeconds = (ltime % 100) * 60;
long secondsOfDay = hoursAsSeconds + minsAsSeconds;
System.out.println("testdate : "+ testdate +", timehrs: "+timehrs+" ,secondsOfDay: "+secondsOfDay);
System.out.println("Orig Date time formatted: "+ new SimpleDateFormat("yyyy-mm-dd hh:mm:ss").format(date));
Date dt = new Date(date.getTime() + secondsOfDay*1000);
System.out.println("New Date : "+ new SimpleDateFormat("yyyy-mm-dd hh:mm:ss").format(dt);
testdate : 2015/10/25, timehrs: 930, secondsOfDay: 34200
Orig Date time formatted: 2015-10-25 12:10:00
New Date : 2015-40-25 09:40:00
Expected : 2015-10-25 09:30:00
Based on below by #Basil Bourque
long nanosOfDay = TimeUnit.MILLISECONDS.toNanos( secondsOfDay*1000 );
LocalTime lt = LocalTime.ofNanoOfDay( nanosOfDay );
ZoneId z = ZoneId.systemDefault();
ZonedDateTime zdt = ZonedDateTime.of( ld , lt , z );
System.out.println("ZonedDateTime zdt: "+ zdt);
ZonedDateTime zdt: 2015-10-25T09:30-07:00[America/Los_Angeles]
This is the correct answer: 2015-10-25T09:30

Answer 1

Question is convoluted
You say you have time of day as a count of milliseconds since midnight (apparently). Yet 930 would mean the time 00:00:00.930, not 09:30:00 as shown in your example data. I will follow your text rather than your example data.
java.time
You are using troublesome old date-time classes, now legacy, supplanted by the java.time classes. Forget you ever heard of java.util.Date and .Calendar.
LocalDate
The LocalDate class represents a date-only value without time-of-day and without time zone.
A time zone is crucial in determining a date. For any given moment, the date varies around the globe by zone. For example, a few minutes after midnight in Paris France is a new day while still “yesterday” in Montréal Québec.
ZoneId z = ZoneId.of( "America/Montreal" );
LocalDate today = LocalDate.now( z );
ISO 8601
The java.time classes use the ISO 8601 standard by default when parsing/generating strings representing date-time values. To make your input string standard, replace those slash characters with hyphens.
String input = "2015/10/25".replace( "/" , "-" );
LocalDate ld = LocalDate.parse( input );
By the way, this whole scheme you have been assigned is awkward, error-prone, and needlessly complicated. To serialize a date-time value for communication between systems, use the ISO 8601 string formats.
LocalTime
You say you have a count of milliseconds to represent the time of day as a count from midnight.
The LocalTime class offers factory methods to instantiate based on a duration of whole seconds and of nanoseconds. To get nanoseconds, simply multiply your milliseconds by one thousand. Better yet, let the TimeUnit enum do the work and make your code more self-documenting.
long millisOfDay = Long.parseLong( "930" );
long nanosOfDay = TimeUnit.MILLISECONDS.toNanos( millisOfDay ); // Same effect as: ( millisOfDay * 1_000L )
LocalTime lt = LocalTime.ofNanoOfDay( nanosOfDay );
ZonedDateTime
Now combine these two Local… objects along with a time zone to determine a point on the timeline. Was this date and time meant to be a moment in Montréal Québec, Paris France, Kolkata India, or Aukland New Zealand?
ZoneId z = ZoneId.of( "Pacific/Auckland" );
ZonedDateTime zdt = ZonedDateTime.of( ld , lt , z );
See live code in IdeOne.com.
ld.toString(): 2015-10-25
lt.toString(): 00:00:00.930
zdt.toString(): 2015-10-25T00:00:00.930+13:00[Pacific/Auckland]
LocalDateTime
If your data came with no information about time zone, and you cannot safely assume the intended time zone by your business scenario, you are left with no better option than combining into a LocalDateTime object. But keep in mind that this value is ambiguous. This value is not a point on the timeline. This value represents potential points on the timeline which can only be determined with a time zone assigned for ZonedDateTime or a offset-from-UTC assigned for OffsetDateTime.
LocalDateTime ldt = LocalDateTime.of( ld , lt );
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.
The Joda-Time project, now in maintenance mode, advises migration to java.time.
To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.
Where to obtain the java.time classes?
Java SE 8 and SE 9 and later
Built-in.
Part of the standard Java API with a bundled implementation.
Java 9 adds some minor features and fixes.
Java SE 6 and SE 7
Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
Android
The ThreeTenABP project adapts ThreeTen-Backport (mentioned above) for Android specifically.
See How to use….
The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

Answer 2

something like:
Date dt = new Date(System.currentTimeMillis() + ms);
or
Date otherDate(....);
Date dt = new Date(otherDate.getTime() + ms);