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How to convert a D M Y HH:MM:SS string in Java to Julian Date?

I have some strings in Java that come in the format: Day Month Year Hour:Minute:Second
7 Jan 2010 23:00:00.000
4 Feb 2010 17:40:00.000
What is the easiest way to parse this string and convert the values to their resulting Julian Dates? I am reading in these strings from Excel so they are not objects with any sort of conversion/formatting utilities (just raw strings). Is there an easy library or function to call to convert these, or would I have to manually write a parser? Thanks

java.time
Sure, Java has got a parser for date and time built-in, the DateTimeFormatter class (named so because it can also format date and time back to strings). And a number of classes that can utilize it for producing objects of themselves. In your case you need the LocalDateTime class. A LocalDateTime is a date and time of day without time zone or offset from UTC, so appropriate for holding the data from your string.
This formatter s good for your string:
private static final DateTimeFormatter FORMATTER
= DateTimeFormatter.ofPattern("d MMM uuuu HH:mm:ss.SSS", Locale.ENGLISH);
Edit: You wrote in a comment:
Plugging in Jan 7 2010 hour 23 into this calculator:
aavso.org/jd-calculator gives
back 2455204.45833. Would this be the exact Julian Date? I believe
your solution was giving the Day instead of Date decimal value
Yes, that’s exactly true. The modified code to get the julian date including the fraction is:
String source = "7 Jan 2010 23:00:00.000";
LocalDateTime ldt = LocalDateTime.parse(source, FORMATTER);
// Subtract half a day to compensate for the
// fact that the Julian day begins at noon
LocalDateTime dateToUseForJulianDay = ldt.minusHours(12);
long julianDayNumber = dateToUseForJulianDay.getLong(JulianFields.JULIAN_DAY);
double juianDateFraction = (double) dateToUseForJulianDay.getLong(ChronoField.NANO_OF_DAY)
/ (double) Duration.ofDays(1).toNanos();
double julianDate = julianDayNumber + juianDateFraction;
System.out.println("Julian date: " + julianDate);
And the output is in this example:
Julian date: 2455204.4583333335
It agrees very nicely with thee result you quote from the online calculator.
The Julian day number is the day number since January 1, 4713 BC. The Julian day starts at noon, which Java does not take into account, so as a hack I have subtracted 12 hours to compensate and get the correct day for all times of day. Since the getLong() method only gets the Julian day number as a whole number, I need to find the fraction separately. It’s a matter of dividing the nanosecond of the day by the total number of nanoseconds in a day. From the original date and time we would have needed the number of nanos since 12 noon; but since I have already subtracted 12 hours, the nanosecond of the day, since 0:00 midnight, is the number we need.
Further link: Julian day on Wikipedia

My library Time4J supports Julian Dates out of the box.
ChronoFormatter<PlainTimestamp> f =
ChronoFormatter.ofTimestampPattern(
"d MMM uuuu HH:mm:ss.SSS", PatternType.CLDR, Locale.ENGLISH);
Moment j2000 = f.parse("7 Jan 2010 23:00:00.000").atUTC(); // are your timestamps really UTC?
// eventually also: ".in(Timezone.ofSystem());"
System.out.println(JulianDay.ofSimplifiedTime(j2000)); // programmer's standard
// JD(POSIX)2455204.4583333335
System.out.println(JulianDay.ofEphemerisTime(j2000)); // astronomical definition
// JD(TT)2455204.459099352
Advantages:
No complex calculation of your owns.
Support for the astronomical definition on the time scale TT.
Explicit display of time zone dependencies (whatever you choose).

java.time.DateTimeException: Invalid value for Year (valid values -999999999 - 999999999)

I am getting this exception when I am trying to convert epochTime to LocalDate where:
1) Date : 2017-05-05 10:08:52.0
2) corresponding epoch :1493959132000
LocalDate lastUpdatedDate = LocalDate.ofEpochDay(1493959132000);
Exception :
java.time.DateTimeException: Invalid value for Year (valid values -999999999 - 999999999): 4090323145
at java.time.temporal.ValueRange.checkValidIntValue(ValueRange.java:330)
at java.time.temporal.ChronoField.checkValidIntValue(ChronoField.java:722)
at java.time.LocalDate.ofEpochDay(LocalDate.java:341)
I understand that the sourcecode of java.time.LocalDate gives a prior warning of this exception at https://docs.oracle.com/javase/8/docs/api/java/time/LocalDate.html#ofEpochDay-long-
What does this actually mean and when does it come?

Here's javadoc of ofEpochDay, this is what it says:
This returns a LocalDate with the specified epoch-day. The EPOCH_DAY
is a simple incrementing count of days where day 0 is 1970-01-01.
Negative numbers represent earlier days.
So, it expects the argument to be number of days since 1970-01-01. As the value you are passing is not valid, it throws the Exception.
Now, coming to your use case, if you want to convert epoch time to localdate then you need to use ofEpochMilli method of Instant class, e.g.:
LocalDate localDate =
Instant.ofEpochMilli(1493959132000l).atZone(ZoneId.systemDefault()).toLocalDate();
System.out.println(localDate);
Here's javadoc for Instant class.
Update
Alternatively, you can convert the timestamp into number of days since 1970-01-01 and pass it to ofEpochDay() method, e.g.:
LocalDate localDate = LocalDate.ofEpochDay(1493959132000l/(1000 * 60 *60 * 24));
System.out.println(localDate);

If you think about it, it doesn't make sense convert epoch seconds to local dates. At a specific moment in time you still need a location/time zone to determine which side of the date demarcation line you're on.
The epoch you mention is Fri, 05 May 2017 04:38:52 UTC. If you're in Greenwhich the LocalDate would be May 5th but if you're on the US West Coast it's still May 4th. Here's a list converting that epoch to different time zones
Instant.ofEpochMilli(1493959132000L).atZone(ZoneOffset.UTC).toLocalDate() //2017-05-05
Instant.ofEpochMilli(1493959132000L).atZone(ZoneId.of("America/Chicago")).toLocalDate() //2017-05-04
Therefore the argument asks for the amount of epoch days to convert to a local date.
LocalDate.ofEpochDay(17291) //2017-05-05

You are using the timeInMillis as the value instead of number of days. You could use the Duration class to calculate the number of days.
Duration duration = Duration.ofMillis(1493959132000);
long days = duration.toDays();
LocalDate lastUpdatedDate = LocalDate.ofEpochDay(days);

Java: Compare Date.getTime with a different date [duplicate]

This question already has answers here:
How to convert date time from one time zone to another time zone
(5 answers)
Closed 7 years ago.
There is the date date1 given with the format YY-MM-dd hh:mm:ss.SSS
I want to compare
date1.getTime()
with one retrieved by doing
new Date().getTime()
There is
SimpleDateFormat sf = new SimpleDateFormat("YY-MM-dd hh:mm:ss.SSS");
Date date1 = sf.parse(date1AsString);
...
compare date1.getTime() with new Date().getTime();
How can I bring these two dates to a common 'timezone' to compare them?
How can I obtain date1 to be on the same 'time length' as new Date()? I want to have the same timezone...
Thanks

From the javadoc of Date
The class Date represents a specific instant in time, with millisecond precision.
An instance in time is agnostic of our definition of time enhanced with time zones. Right now is the same for you and me, regardless of the fact that we are (potentially) in different time zones.
What adds the notion of a time zone is the DateFormat
The date is represented as a Date object or as the milliseconds since January 1, 1970, 00:00:00 GMT.
When you invoke Date#getTime(), you get back
the number of milliseconds since January 1, 1970, 00:00:00 GMT represented by this Date object.
This is something you can use to compare Date objects since they have the same root. Similarly, the compareTo will return
the value 0 if the argument Date is equal to this Date; a value
less than 0 if this Date is before the Date argument; and a value
greater than 0 if this Date is after the Date argument.

You cannot compare two dates if their timezone information is unknown.
For example if your date1AsString variable is simply 2014-12-16 16:00:00 then you cannot tell if it is greater than or less than 2014-12-16 20:00:00+0000.
It looks like first date is 4 hours less than second one; but if someone adds that the first date is Pacific Time (UTC-0800) then it would actually be 4 hours more than the second date (2014-12-16 16:00:00-0800 = 2014-12-17 00:00:00+0000).
So, if date1AsString has an unknown timezone then you cannot convert it to UTC or anything else for comparison.

getTime() Returns the number of milliseconds since January 1, 1970, 00:00 :00. You will want to compare the two longs, roughly like so:
long time = new Date().getTime();
long time2 = date1.getTime();
if(time>time2)
System.out.println(String.format("time is greater than time2 by %d", time-time2)
else
System.out.println(String.format("time2 is greater than time by %d", time2-time)

if you know Timezone
date1.setTime(date1.getTime()+/-(3600000*hours)) to convert, if date1 time is GMT to go to GMT+2 hours must be 2 and sing must be +

Typically any server environment is going to be using GMT so you wont have to worry about the timezone when comparing dates, only when formatting the value to be display to users.
You can compare two dates like so:
SimpleDateFormat sf = new SimpleDateFormat("YY-MM-dd hh:mm:ss.SSS");
Date date1 = sf.parse(date1AsString);
boolean isBefore = date1.before(new Date());
boolean isAfter = date1.after(new Date());
Alternatively, you can compare the numeric values of the unix time to get the same result:
...
boolean isBefore = date1.getTime() < new Date().getTime();
boolean isAfter = date1.getTime() > new Date().getTime();

The Date Class has methods like before(date) and after(date). You should take a look into the Date Class.

So I think what you are looking for is to set the time zone of the SimpleDateFormat:
sf.setTimeZone(TimeZone.getTimeZone("EST"));

Having problems on getting actual time in UTC Android

I am trying to get the the day and time in UTC as milliseconds but repeatedly get the same problem. The result should be something like '63530139420000' but each time the value '1394547490884' is returned.
To get the date and time in UTC I use the following method:
long dateutc = System.currentTimeMillis();
Can anyone tell me what is the problem?

You can use this.
DateFormat df = DateFormat.getTimeInstance();
df.setTimeZone(TimeZone.getTimeZone("UTC"));
String dateutc = df.format(new Date());

Your code correctly gets the number of milliseconds since January 1, 1970 00:00:00 UTC.
Interestingly if you take the value your teacher has specified and divide by the number of milliseconds in a year you get 2014.5. So either your teacher doesn't know what currentTimeMillis() does or he wants milliseconds since year 0 (which doesn't make any sense to me).
To calculate number of milliseconds since a given date all you need to do is to create two date instances and subtract the milliseconds values from getTime().

How can I parse a Date String with the format "\/Date(1339638938087-0500)\/" in Java?

I have String with the format "\/Date(1339638938087-0500)\/" from a web service api.
Using java, how can I put this into a org.joda.time.DateTime variable?

You need to extract these two bits of information:
-2208967200000: milliseconds since the unix epoch (January 1st 1970 UTC)
-0600: offset from UTC
This assumes that the example represents January 1st 1900 at midnight local time - as the -2208967200000 part represents 6am UTC.
To convert this into a Joda Time DateTime, you should extract the two parts (get rid of everything outside the brackets, and then either use the length to split it, or find the middle +/- symbol).
Next, parse the first part as a long for the millis section.
Then, parse the second part - probably as hours, minutes and sign separately. (I'm assuming it's always in the form xHHmm where x is the sign, HH is the minutes as two digits, and mm is the hours as two digits.)
Create a fixed time zone for the offset using DateTimeZone.forHoursMinutesOffset or something similar.
Finally, create the time with
new DateTime(millis, zone);
Oh, and then kick whoever's producing such a horrible format...

this look like unix timestamp
The unix time stamp is a way to track time as a running total of
seconds. This count starts at the Unix Epoch on January 1st, 1970

If the "2208967200000" is a time in milliseconds since January 1, 1970, 00:00:00, you can use it in constructor for Date(time);
String dateStr="Date(-2208967200000-0600)";
String timeInMillis=dateStr.Split("-")[1];
String utcStr=dateStr.Split("-")[2].Substring(0,4);
Date d=new Date(Long.parseLong(timeInMillis));
if you want you can handle utcStr if it is necessary (if the second part after "-" is a time zone)