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I am very new to velocity apache script

Present code is am using is like this:
#set($startdate=$datetool.format("yyyymmdd",$fromDate))
#set($enddate=$datetool.format("yyyymmdd",$endDate))
But I dont know how to continue.
I want to find out difference between two dates which are in the format yyyymmdd, and if the difference is greater than one month, I have to divide that into months, so please anyone help to solve this.

I Would suggest you to to use ComparisonDateTool in Velocity to get date difference
$dateComparisonTool.difference($startdate,$enddate).days

How to convert nano second to seconds?

Is there a way to convert the event.timestamp to regular metrics. My current output is 267185117663619 (nanosecond??). Is there a way to conver this to, let's say miliseconds or even seconds?
timestamp = String.valueOf(event.timestamp);
Thanks in advance!

Well, you could just divide it by 1,000,000,000 .
This way:
double seconds = (double)Long.parseLong(timestamp)/1000000000.0;

JavaScript formatting time 24hours format to 12hours

I was having some trouble when trying to format time in 24 hours format to 12 hours format. Here are some of the example of my time in string format:
0:00, 9:00, 12:00, 15:00
I wonder how should I substr the first two character in JavaScript because some of them were one digit and some were two. The output time format should be in 12 hours format like:
12:00AM, 9:00AM, 12:00PM, 3:00PM
Any guides? Thanks in advance.

In comments you clarified that each string you process will have only a single time in it (i.e., you are not processing a single string with four comma-separated times in it). So essentially you have input as follows:
var input = "9:00";
The easiest way to extract the hour and minute is using the String .split() method. This splits up the string at a specified character - in your case you'd use ":" - and returns an array with the pieces:
var parts = input.split(":"),
hour = parts[0],
minute = parts[1];

The obvious answer would be to use regular expressions (but remember AWZ's rule: if you have a problem and decide it can be solved with RE's, then you now have two prolems).
However, save yourself a whole helluva lot of trouble and get moment.js

Time of the day in minutes Java

I am looking to calculate the number of minutes given the time of the day.
Eg.: when input is 11:34, the output should be 11*60+34. The date doesn't matter.
I only need it down to the minutes scale. Seconds, milliseconds... don't matter.
Is there a method somewhere in Java doing this the neat way without me calculating it?
Right now, i'm using theTime.split(":"), theTime is a String holding "11:34" here, parsing the integers on each side and doing the calculation.
I saw Time but what I'm doing right now seemed more direct.
Nothing in Systems either.

There is no build in method for it. However here is a one-liner for it:
int timeInMins = Calendar.getInstance().get(Calendar.HOUR_OF_DAY) * 60 + Calendar.getInstance().get(Calendar.MINUTE);

Your approach looks good and sound, however to answer your question it would be simple to say that there is no such build in method which does that. You have to calculate it the way you are doing it right now.

Hi maybe you could use JodaTime? Below example how to get number of minutes from parsed string and from current time. In java 8 there is similar api but I haven't found exactly method like minutesOfDay()
#Test
public void learnHowManyMinutesPassedToday() {
DateTime time = DateTimeFormat.forPattern("HH:mm").parseDateTime("11:34");
System.out.println(time.getMinuteOfDay());
System.out.println(DateTime.now().getMinuteOfDay());
}

If you are looking to have input not from a String, take a look at
Java.util.Calendar.
It has Calendar.HOUR_OF_DAY and Calendar.HOUR and Calendar.MINUTE which could be your input. I'm not sure what the "neat" way of doing this would be. It is a simple calculation.

Calendar rightNow = Calendar.getInstance();
int hour = rightNow.get(Calendar.HOUR);
int min = rightNow.get(Calendar.MINUTE);
System.out.println("TimeMinutes:" + hour * 60 + min);
EDIT:
Except using split use the above.

How to format numbers on Android

i am learning to program mobile aplications on Android. My first app is a unit converter. Everithing is working for now, but i have a question about formating numbers. I hava this code to get text from buttons and to convert the appropriet output:
if (bPrevodZ.getText() == "milimeter"){
if (bPrevodDo.getText()=="kilometer"){
String PomocnaPremenna = jednotkaZ.getText().toString();
double cisloNaPrevod = Double.parseDouble(PomocnaPremenna);
cisloNaPrevod = cisloNaPrevod*0.0000001;
vysledok.setText(Double.toString(cisloNaPrevod));
}
The final result is "cisloNaPrevod", but i have problems to show a good format of that number. For example:
12345 mm = 0,0012345 km this is good right ? :)
but if i convert:
563287 mm = 0.05632869999999995 this is bad :) i need it to show 0.0563287
Thx for any help

Use String.format:
String.format("%.6f", cisloNaPrevod);

If you want your number to always have 6 significant figures, use
vysledok.setText(String.format("%.6g", cisloNaPrevod));
giving the result 0.0563287.
If you want to round to 6 numbers after the decimal place, use
vysledok.setText(String.format("%.6f", cisloNaPrevod));
giving the result 0.056329.
Here's some good resources that cover number formatting:
Floating-point cheat sheet for Java
java.util.Formatter

If it's something you're going to do often, perhaps you should use DecimalFormat.
DecimalFormat df = new DecimalFormat("#.#######");
Then call:
df.format(someDoubleValue);