date calculations

[mtissington]

Is it possible to do date/time calculations like "today + 5d" - which would return todays date + 5 days.

If not, any suggestions?

Thanks.

[mariuszgromada]

Hello @mtissington ,

Thank you for this interesting question. mXparser is a numerical library, it does not work directly with other data types such as text or data. However, mXparser is very flexible and you can address your requirement by using extensions. If we assume that the date e.g. 2025-03-08 can be represented as a number e.g. int 20250308, then such operations on numbers can be safely performed using simple written extensions. Here is an example of such an implementation for java. For other languages, please refer to docs.

User defined arguments
Case 3: Implementing your own Argument Extension
https://mathparser.org/mxparser-tutorial/user-defined-arguments/

User defined functions
Case 5: Implementing your own Function Extension
https://mathparser.org/mxparser-tutorial/user-defined-functions/

Variadic user defined functions
Case 5: Implementing your own Variadic Function Extension
https://mathparser.org/mxparser-tutorial/variadic-user-defined-functions/

Example extension implementation to operate on dates and today.

import org.mariuszgromada.math.mxparser.*;
import java.time.LocalDate;

class Today implements ArgumentExtension {

	@Override
	public double getArgumentValue() {
		LocalDate today = LocalDate.now();
		int year = today.getYear();
		int month = today.getMonthValue();
		int day = today.getDayOfMonth();
		return year * 10000 + month * 100 + day;
	}

	@Override
	public ArgumentExtension clone() {
		return new Today();
	}

}

class DateAddDays implements FunctionExtension {
	private int dateAsNum;
	private int daysToAdd;

	@Override
	public int getParametersNumber() {
		return 2;
	}

	@Override
	public void setParameterValue(int parameterIndex, double parameterValue) {
		switch (parameterIndex) {
			case 0: dateAsNum = (int)parameterValue;
				return;
			case 1: daysToAdd = (int)parameterValue;
				return;
		}
	}

	@Override
	public String getParameterName(int parameterIndex) {
		switch (parameterIndex) {
			case 0: return "dateAsNum";
			case 1: return "daysToAdd";
		}
		return "";
	}

	@Override
	public double calculate() {
		int year = dateAsNum / 10000;
		int monthDay = dateAsNum % 10000;
		int month = monthDay / 100;
		int day = monthDay % 100;

		LocalDate date = LocalDate.of(year, month, day);
		LocalDate newDate = date.plusDays(daysToAdd);

		int newYear = newDate.getYear();
		int newMonth = newDate.getMonthValue();
		int newDay = newDate.getDayOfMonth();

		return newYear * 10000 + newMonth * 100 + newDay;
	}

	@Override
	public FunctionExtension clone() {
		return new DateAddDays();
	}
}

public class DateTests {

	public static void main(String[] args) {
		Argument today = new Argument("tdy", new Today());
		Function dtadd = new Function("dtadd", new DateAddDays());

		Expression e1 = new Expression("dtadd(tdy, 5)", today, dtadd);
		Expression e2 = new Expression("dtadd(20250101, 365)", dtadd);

		System.out.println( (int)e1.calculate() );
		System.out.println( (int)e2.calculate() );
	}
}

Code result

20250313
20260101

Best regards

[mtissington]

Thank you for this - It gives me some ideas to explore. A couple of basic questions

If I want to support things like

today, year, now, tomorrow etc.

Would I implement these as Constants? I need something that would be evaluated at calculation time.

So then I might be able to implement something like "today + 1w" and would return the date of one week from today.

Can your library return a string instead of a number?

[mariuszgromada]

The library returns only numbers and works only on numbers. The code I wrote for you shows how to do addition using a function. You can't do this with the + operator, because it will do numeric addition.

[mtissington]

I understand that - so I have to return a date/time in numerical format - I can do that.

But if I can do "2 + pi" can I write my own constants and be able to do something like "2 + today" ??
If I have defined "today" as a Constant.

And when the Calculate method is called I return a number for "today"

[mariuszgromada]

It will do the addition, but the result will not be correct,

20250328 + 10 = 20250338

20250338 - does not exists as a date. It has to be the extended function as then you can put your code in the function extension, as I did in the initial example.

Best regards

[mariuszgromada]

Dear @mtissington

I have found a solution for you, please take a look on the result below:

Expression e1 = new Expression("tdy", today, date);
Expression e2 = new Expression("date(20250101) + 365", date);

System.out.println( e1.getExpressionString() + " = " + Date.toString(e1.calculate()) );
System.out.println( e2.getExpressionString() + " = " + Date.toString(e2.calculate()) );

tdy = 2025-03-09
date(20250101) + 365 = 2026-01-01

And the full code:

import org.mariuszgromada.math.mxparser.*;
import java.time.LocalDate;
import java.time.temporal.ChronoUnit;

class Date implements FunctionExtension {
	static final LocalDate UNIX_EPOCH_DATE = LocalDate.of(1970, 1, 1);
	private int dateAsNum;

	@Override
	public int getParametersNumber() {
		return 1;
	}

	@Override
	public void setParameterValue(int parameterIndex, double parameterValue) {
		switch (parameterIndex) {
			case 0: dateAsNum = (int)parameterValue;
				return;
		}
	}

	@Override
	public String getParameterName(int parameterIndex) {
		switch (parameterIndex) {
			case 0: return "dateAsNum";
		}
		return "";
	}

	@Override
	public double calculate() {
		int year = dateAsNum / 10000;
		int monthDay = dateAsNum % 10000;
		int month = monthDay / 100;
		int day = monthDay % 100;

		LocalDate date = LocalDate.of(year, month, day);
		return ChronoUnit.DAYS.between(UNIX_EPOCH_DATE, date);
	}

	@Override
	public FunctionExtension clone() {
		return new Date();
	}

	public static String toString(double dateAsDaysSinceEpoch) {
		LocalDate date = UNIX_EPOCH_DATE.plusDays((long)dateAsDaysSinceEpoch);
		return date.toString();
	}
}


class Today implements ArgumentExtension {

	@Override
	public double getArgumentValue() {
		LocalDate today = LocalDate.now();
		return ChronoUnit.DAYS.between(Date.UNIX_EPOCH_DATE, today);
	}

	@Override
	public ArgumentExtension clone() {
		return new Today();
	}

}


public class WorkingTests {

	public static void main(String[] args) {
		Argument today = new Argument("tdy", new Today());
		Function date = new Function("date", new Date());

		Expression e1 = new Expression("tdy", today, date);
		Expression e2 = new Expression("date(20250101) + 365", date);

		System.out.println( e1.getExpressionString() + " = " + Date.toString(e1.calculate()) );
		System.out.println( e2.getExpressionString() + " = " + Date.toString(e2.calculate()) );
	}
}

Basically what I did is a conversion to number of days since unix epoch.

Best regards

[mtissington]

Really appreciate this - thank you ... I still need to think about it more.

It maybe too complicated for the user to use function like 'date()'. Maybe not
I guess I can parse what ever is inside the date function so date("10/20/2025") would work?

My aim is something like 'yesterday + 90d' or 'today + 1w' or 'tomorrow + 1y' or 'date(10/20/2025) + 2m'
Here d, m, y, w could be setup as constants?

I'm surprised I can't find a library that already does this.

Again thank you for the detailed help and ideas :)

[mariuszgromada]

yesterday can be done as an extended argument, similarly to the "today" example I provided above, then "w" and "y" can be just a constant w = 7, y = 365. Unfortunately date("10/20/2025") will not work as grammar is not prepared to parse "10/20/2025".