How to work out days weeks months years between dates ?

[Ray mann]

Hi guys,
so i would like to know how to work out days weeks months years between dates ? Is there a java class that can do this for me as there are not 30 days in every year and not 365 days in every year.
This is my attempt so far, i know there is a not exactly 4 weeks in a months !  what is the correct way to do this ?  All help and advice is greatly appreciated.

public class Days{
	public static void main(String[]args){
	int days =   32; ///this was just a test case 
	int weeks =  0;
	int months = 0;
	int years =  0;

	//days in a week
	if (days>=7){
		weeks = days/7;
		days = days%7;
	}//weeks in a month NOT EXACTLY
	if (weeks>=4){
		months = weeks/4;
		weeks = weeks%4;
	}//months in a year
	if(months>=12){
		years = months/12;
		months = months%12;
	}
System.out.print("I am " );
//day vs days
	if (days == 0)
		System.out.print("");
	else if (days == 1)
		System.out.print(days + " day ");
	else
		System.out.print(days + " days ");
///week vs weeks
	if (weeks == 0)
		System.out.print("");
	else if (weeks == 1)
		System.out.print(weeks + " week ");
	else
		System.out.print(weeks + " weeks ");
//month vs months
	if (months == 0)
		System.out.print("");
	else if (months == 1)
		System.out.print(months + " month ");
	else
		System.out.print(months + " months ");
///year vs years
	if (years == 0)
		System.out.print("");
	else if (years == 1)
		System.out.print(years + " year ");
	else
		System.out.print(years + " years ");

		System.out.print( " old \n");



	}


}

Thanks in advance !

[Tim Moores]

The java.time API (available from Java 8 onwards) has all kinds of goodies you can use, but for the basic days-between-dates calculation check out the neat formula called Zeller's congruence.

[Paul Clapham]

If you're really going to try seriously to answer that question, you're going to need to decide what it means before you get too far along. For example:

How many months are there between December 30, 2016 and January 2, 2017? That could plausibly be zero, one, or two depending on what rules you decide to apply. You can easily think up similar test cases for weeks and years, but they would all likely have the same sort of answer depending on what you do with partial weeks, months, and years.

[Ray mann]

I solved the problem with the following method, here is the code :

///NB dont forget the import: import java.time.*;


public static void myDays( LocalDate startLocalDate , LocalDate CurrentLocalDate){

		Period p = Period.between(startLocalDate, CurrentLocalDate);

		System.out.print(" ,so I am ");
		//print years >0
		if (p.getYears()==0)
			System.out.print("" );
		else
			System.out.print( p.getYears() + " years, " );
		//print months > 0
		if (p.getMonths()==0)
			System.out.print( "" );
		else
			System.out.print( p.getMonths() + " months, " );
			//print day >0
		if (p.getDays()!= 0)
			System.out.print( p.getDays() + " Days " );
		else
			System.out.print("");

		System.out.print("  old. \n");

	}

Thanks guys!

[Campbell Ritchie]

Nice to see how two local dates can form a period
There are however all sorts of things which could be improved there.
Why have you got those if blocks which do nothing? You can change them all to whatever is in the else part.
Don't print an empty String: what is the point of it. Don't use \n unless somebody has told you they require LF. Use println instead.
It is probably better not to use *s in import statements.

[Ray mann]

I made some changes to the code:

//prints years months days
	public static void myDays( LocalDate startLocalDate , LocalDate CurrentLocalDate){

		Period p = Period.between(startLocalDate, CurrentLocalDate);

		System.out.print(" ,so I am ");
		//print years >0
		if (p.getYears()==1){
				System.out.print( p.getYears() + " year, " );
		}else if (p.getYears()>1){
				System.out.print( p.getYears() + " years, " );
		}//print months > 0
		if(p.getMonths()==1){
				System.out.print( p.getMonths() + " month, " );
		}else if(p.getMonths()>1){
				System.out.print( p.getMonths() + " months, " );
		}	//print day >0
		if (p.getDays()!= 0){
			if(p.getDays()== 1)
				System.out.print( p.getDays() + " Day " );
			else
				System.out.print( p.getDays() + " Days " );
		}
		System.out.print("old. ");
		System.out.println();

	}

Thanks for the advice, I realized there was no point to the else block doing nothing, as thats the same as having no else block vice versa.

[Tony Docherty]

Some further advice, albeit more of the nit picking kind.

The Java naming convention is to start variable names with a lowercase letter so 'CurrentLocalDate' should be 'currentLocalDate'.
Be consistent in your formatting: you've added spaces in some places and not others eg if (p.getYears()==1) and if(p.getMonths()==1)
Be consistent in your code: you've changed the structure of the if statements for the last case (days) which means it does something slightly different to the other 2, ie the first 2 if statements ignore negative values the last one prints out a negative value.

Also you may want to add special case handling for if the period is negative or the two dates are exactly the same.

[Ray mann]

Here are the further changes made to the code :

//Prints years/months/days and other output
public static void myDays( LocalDate startLocalDate , LocalDate currentLocalDate, String creationDateFormat){
		Period p = Period.between(startLocalDate, currentLocalDate );
		//Neg case
		if (p.getYears()<0 || p.getMonths()<0 || p.getDays()<0){
				System.err.println("Current system time precedes the programs installation date. Please set system  date and time correctly.");
				System.exit(1);
			}
		//Equal case/Same Date
		if (startLocalDate.equals(currentLocalDate)){
			System.out.println("I was initialized Today");
				System.exit(0);
			}
		System.out.print("I was initialized on " + creationDateFormat + ", so I am ");

		//Print year vs years
		if (p.getYears()==1){
				System.out.print(p.getYears() + " year, ");
		}else if (p.getYears()>1){
				System.out.print(p.getYears() + " years, ");
		}//Print month vs months
		if(p.getMonths()==1){
				System.out.print(p.getMonths() + " month, ");
		}else if(p.getMonths()>1){
				System.out.print(p.getMonths() + " months, ");
		}//Print day vs days
		if (p.getDays()==1){
				System.out.print(p.getDays() + " Day ");
		}else if (p.getDays()>1){
				System.out.print(p.getDays() + " Days ");
		}
		System.out.print("old. ");
		System.out.println();

	}

Thanks for the advice and criticism, all valuable!  

[Campbell Ritchie]

There is a lot of repetition in that code. You are testing for == 1 thrice. That shou‍ld factored out into a method by itself.

[Ray mann]

Factored repeated if block to its own method :

//Prints neg and equal case and calls timePreiods(int,String).
	public static void myDays( LocalDate startLocalDate , LocalDate currentLocalDate, String creationDateFormat){
		Period p = Period.between(startLocalDate, currentLocalDate );
		//Neg case
		if (p.getYears()<0 || p.getMonths()<0 || p.getDays()<0){
				System.err.println("Current system time precedes the programs installation date. Please set system  date and time correctly.");
				System.exit(1);
			}
		//Equal case/Same Date
		if (startLocalDate.equals(currentLocalDate)){
			System.out.println("I was initialized Today");
				System.exit(0);
			}
		System.out.print("I was initialized on " + creationDateFormat + ", so I am ");
		
                timePreiods(p.getYears() ," year");
		timePreiods(p.getMonths() ," month");
		timePreiods(p.getDays() ," day");

		System.out.print("old. ");
		System.out.println();

	}
	//Prints int timeAmout string years/months/days
	public static void timePreiods(int periodsAmount ,String periodsName){
		if (periodsAmount==1){
			System.out.print(periodsAmount + periodsName+" ");
		}else if (periodsAmount>1){
			System.out.print(periodsAmount + periodsName +"s ");
		}

	} 

Very Clever...
Thanks. I am learning from experienced devs here feeling fortunate!        

[Campbell Ritchie]

Well done
You can shorten that method with the ?: operator.
Don't use System.exit. Use some if‑elses and let the method terminate normally instead.

[Ray mann]

Awesome thanks further changes :

//converts Date to LocalDate type
	public static LocalDate fromDate(Date date) {
		Instant instant = Instant.ofEpochMilli(date.getTime());
		return LocalDateTime.ofInstant(instant, ZoneId.systemDefault()).toLocalDate();

	}
	//Prints neg and equal case and calls timePreiods(int,String).
	public static void myDays( LocalDate startLocalDate , LocalDate currentLocalDate, String creationDateFormat){
		Period p = Period.between(startLocalDate, currentLocalDate );
		//Neg case
		if (p.getYears()<0 || p.getMonths()<0 || p.getDays()<0){
			System.err.println("Current system time precedes the programs installation date. Please set system  date and time correctly.");
			//Equal case/Same Date
		}else if (startLocalDate.equals(currentLocalDate)){
			System.out.println("I was initialized Today");
		}else{
			System.out.print("I was initialized on " + creationDateFormat + ", so I am ");
			timePreiods(p.getYears() ," year");
			timePreiods(p.getMonths() ," month");
			timePreiods(p.getDays() ," day");
			System.out.print("old. ");
			System.out.println();
		}
	}
	//Prints int timeAmout string years/months/days
	public static void timePreiods(int periodsAmount ,String periodsName){
	
		String amountsAndNames = (periodsAmount==1) ? periodsAmount + periodsName+" " : ( (periodsAmount>1 )? periodsAmount + periodsName +"s " : "") ;
		System.out.print(amountsAndNames);
	}

[Campbell Ritchie]

You are making life hard for yourself by not using the methods of LocalDate which allow you to compare them. Have a look at the API documentation.
But your code is improving every time.

[Tony Docherty]

or you could do:

        String amountsAndNames = periodsAmount + periodsName + ((periodsAmount==1) ? " " : "s ") ;