This code lets you calulate the difference in months between two dates, Date2 > Date1
public int MonthDiff(DateTime Date1, DateTime Date2)
{
return Math.Abs((Date2.Month - Date1.Month) + 12 * (Date2.Year - Date1.Year));
}
In my example if I put Date1 = "01/01/2019" & Date2 = "31/12/2019", it will return 11, and this is wrong, it should be 12.
Also if I put Date1 = "25/01/2019" & Date = "31/12/2019", it should be 12.
So the question should I need to calculate by Days or what?
I used this code
return Math.Abs(((Date2- Date1).Days / 30) + 12 * (Date2.Year - Date1.Year));
With Date1 = "01/01/2019" & Date2 = "31/12/2020", it shows 36 Months.
If you want to know the difference between two dates in variable length units like months or years, you should use the built in timespan functionality rather than roll your own, and accept some compromises/approximations. The average number of days in a year is 365.2425. The average number of days in a month is 30.42 for a non leap year or 30.50 for a leap year, or 30.44 overall. Choose one of these values when approximating the months. Choose whether to round down, or round up, and to how many decimal places when working out the months/years
For example:
var a = DateTime.Now;
var b = DateTime.Now.AddDays(366);
var years = Math.Round((b-a).TotalDays/365.2425);
You could take some alternative approaches like:
have an array of integers that depict the number of days in each month (over a four year period so leap February are accounted for) and a logic of "I will get the number of days between the two dates, then I will consider the month of the start date and set an array indexer variable pointing to that month in the a"rray-of-month-lengths", and I will subtract the number of days in that month from my total, then I'll move on to the next index in the array (going back to the array start if necessary) and subtract that.. and I'll keep doing it until the remaining total days is lower than the number of days in whatever month I'm looking at.. and the number of times I looped shall be the result of the number of months between the day"
write a loop that adds one day to the start date until the end date is breached, and count how many times the current month number is different to the month number on the last iteration of the loop
etc
These are decisions to implement very specific sorts of approximations
I don't think there is a good fact based answer to your question until you accurately explain every rule you want your math you work to, so all the exceptions can be coded for. Saying "x - y is 11 months and this is wrong" is not a rule; you need to say why it is wrong
Related
I have a unit test where the expected result is a DateTime and is set as follows:
var expectedResult = DateTime.Today.AddMonths(3).AddMonths(3);
After which I have a function that adds quarters to a date:
DateTime.AddMonths(3 * numberOfTimes);
numberOfTimes is in this case 2.
The results differ in date. Today is 31/01/2023, expected result is 30/07/2023 and the function result is 31/07/2023.
I expected the results to be the same because 6 months should have been equal number of days from the starting date. I am curious why this happens. For now I fixed the problem by doing 3 * numberOfTimes in the expectedResult section.
Just out of curiosity why does this happen?
It is documented:
The AddMonths method calculates the resulting month and year, taking
into account leap years and the number of days in a month, then
adjusts the day part of the resulting DateTime object. If the
resulting day is not a valid day in the resulting month, the last
valid day of the resulting month is used. For example, March 31st + 1
month = April 30th, and March 31st - 1 month = February 28 for a
non-leap year and February 29 for a leap year.
So you get a different result because if you add 6 months it just needs to be checked if 31/07/2023 is a valid DateTime, which it is. For DateTime.Today.AddMonths(3).AddMonths(3) it will check first if 31/04/2023 is valid, which is not, so 30/04/2023 is returned, then 3 months are added.
The result can be different because of the way that the date and time is calculated for each method. The DateTime.AddMonths method takes into account the number of days in each month and can result in a different day of the month if the original date has a different number of days in the month. For example, adding 3 months to January 31st would result in a different day of the month than adding 6 months to January 31st.
I have a start date column called StartDate in a database table. I need to determine how many weeks elapsed from the start date until today.
Here is my code:
DateTime startDate = new DateTime(StartedDate);
if (startDate.addDays(7) == DateTime.Today) {
// One week elapsed.
}
Let's say startDate is 9/29/2016. If I add 7 days, the total becomes 10/7/2016.
If, for example, today is 10/7/2016 - the same date as above, so there is 1 week from the start date. How can I determine the number of weeks for dates in the future?
Try
if(DateTime.Now.Subtract(StartDate).TotalDays%7==0)
This will give you the modulus of days and equal 0 every 7 days. It will, however, be time sensistve (if StartDate is 2:00PM, days will be 6 until 2:00PM on day 7). If you are only concerned about the day (not time after midnight) use:
if(DateTime.Now.Date.Subtract(StartDate.Date).TotalDays%7==0)
I was writing a converter that takes a person's date of birth and produces their age in years. I wrote something that looked like this:
public class DateOfBirthToAgeConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
var date = value as DateTime?;
if (date == null) return null;
return (DateTime.Now - date).Years;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}
I found that there is no Years property on the TimeSpan that results from the subtraction of two DateTime objects. I was somewhat surprised by this. I thought about why there might not be a Years. I figured that it might be because of the leap day, but by that logic, there shouldn't be Days because of daylight savings.
The absence of Months made sense, since there is no standard month length.
I was able to write some different code to get the correct age, but I still really want to know why there is no Years or Weeks property on TimeSpan. Does anyone know the reason?
A TimeSpan only contains the difference between two DateTime values. It is unknown which year this TimeSpan is in. That's also why it doesn't have a Months property.
Example:
TimeSpan.FromDays(60)
How many months are that? 1 or 2?
The absence of Months made sense since there is no standard month length.
There is no standard year length either because of leap years.
Workaround: If you really want to display an approximate value, then doing TimeSpan.TotalDays / 365.2425 will do just fine.
Edit: But only for rough estimations and not for birthdays. In birthday calculation, leap days will accumulate every 4 years as pointed out by Henk Holterman in the comments. Take a look here for calculation of birthdays.
Programmer's life is really hard.
The length of year is variable. Some years have 365 days and some have 366 days. According to the calendar, some years could even have missing days. If talking about culture it becomes more difficult since Chinese lunar calendar can have 13 months a year.
The length of month is variable, and this is well-known. This is also to know that in other calendars things can get worse.
The length of day is variable, because of daylight savings and this is not just culture dependent but also geography dependent.
The length of hour and minute are variable, because of leap seconds.
It seems the only thing that is reliable is the length of a second. So internally, timespan is stored in seconds (or milliseconds, which is the same).
But the variability of time units makes the answer "how many (years/months/days/hours/minites) for n seconds?" being always inaccurate.
This is why the developers end up with a solution that is useful in practical but not precise. They simply ignore daylight savings and leap seconds. However, since people hardly ask about years and months, they just decided not to answer those questions.
Rhetorical question: Without a point of reference, how long is a year?
Because a TimeSpan does not have a fixed point in time, it is not possible to unambiguously say how long a year at an unknown time will be. In the simplest case, it might be 365 or 366 days. There are considerably more cases that would affect the outcome.
I figured that it might be because of the leap day, but by that logic,
there shouldn't be Days because of daylight savings.
You have a point there; subtracting two dates doesn't handle daylight savings ideally. If the dates are local time, you may get an unexpected result.
A change in daylight savings time means a gap or overlap in the local time, and that is ignored if you do calculations with the dates. So, if you want to get the exact difference between two DateTime values that are local time, you should convert them to UTC first as that has linear time:
TimeSpan diff = date1.ToUniversalTime() - date2.ToUniversalTime();
The reason that the TimeSpan doesn't have years is that years differ in length. The daylight savings issue is an effect of how you calculate the TimeSpan and can be circumvented, but there is no "linear years" that you can use to circumvent leap years.
Timespan simply stores number of milliseconds. If you have (1000 * 60 * 60 * 24 * 365.5) 365.5 days worth of milliseconds it's impossible to know if that number of milliseconds spans an entire year and into the next, if it's just short of a year, or if it spans across three years. Same with 30.5 days worth of milliseconds, could span into a second month, could be less than a month, could span across three months.
Assuming you can start with 2 dates and not a single TimeSpan, then you can get the difference in years like this...
The DateTime.AddYears() function has already solved the majority of issues you may encounter with leap years etc. So for a difference in years, start with the earliest of the two dates and repeatedly add 1 year using .AddYears(1) and increment a counter till you have a date which exceeds the later date of the two and the difference in years will be the value of the counter -1.
If you think the 2 dates may be thousands of years apart, you could adapt this logic to repeatedly +100 years, when the later date is exceeded, repeatedly -10 years, when you go below the higher date, finally +1 again till the higher date is exceeded. If you + and - from the counter the number of years you are adding and taking away, you'll end up with counter -1 being the number of whole years difference again.
public int AgeInYears(DateTime date1, DateTime date2)
{
int age = 0;
DateTime low;
DateTime high;
DateTime test;
if(date1 < date2)
{
low = date1;
high = date2;
}
else
{
low = date2;
high = date1;
}
test = low;
while(test < high)
{
test = test.AddYears(100);
age += 100;
}
while (test > high)
{
test = test.AddYears(-10);
age -= 10;
}
while(test <= high)
{
test = test.AddYears(1);
age++;
}
return age - 1;
}
I'm writing a search program that includes a date range- DateFrom and DateTo, both of which are DateTimes. Searching January - April for any search criteria will return the results of January + February + March + April.
I would like to add functionality whereby a user can choose to search each month within a - range, so searching January - April will return each individual month's results. However I'm having trouble finding an intelligent way to implement this for any unit of time larger than days.
So far I'm getting a TimeSpan using:
TimeSpan ts = query.DateTo - query.DateFrom;
In a perfect world I'd just be able to do something like foreach (month m in TimeSpan){dostuff}, however TimeSpan stores dates as integers and does not include any units larger than days. Additionally, I thought maybe I could just use n = DateFrom.month - DateTo.month to get the difference in months and run a function in a for loop starting with DateFrom and lasting n months, but this won't work between years.
The last case is definitely fixable but includes a number of tedious special cases. Is there a cleaner / more elegant way of accomplishing this sort of iteration that I'm missing?
Thanks.
So for the basic pattern we can use a fairly simple for loop:
public static IEnumerable<DateTime> Months(DateTime start, DateTime end)
{
for (DateTime date = start; date < end; date = date.AddMonths(1))
yield return date;
}
Now in this case we have a start date that is inclusive and an end date that is exclusive. If we want to make the end date inclusive, as you have described, we can add:
end = end.AddMonths(1);
to the start of the method.
Next you have a few other considerations. Are the datetime objects passed in going to always be the first of the month? If not, how do you want to support it? If the start date is Feb 10th do you want the first yielded date to be Feb 1st (the start of the start date's month), March 1st (the first "first day of the month" on or after the start date), or Feb 10th (meaning that each date in the timespan would be the 10th day of that month)?
Those same questions also apply to the end date; do you want the last "first day of the month" before the end date, the first day of the next month, etc.
Also, what should happen if the start date is after the end date? Should it yield the dates "backwards", should it just pretend the start date is the end date and the end date is the start date? Should it keep adding days until you've overflowed DateTime and come back around to that date?
Pretty much all of these issues aren't too hard to deal with, the hard part is just knowing what you want to do in each case.
You could do something like:
var months = ((query.DateTo.Year - query.DateFrom.Year) * 12) + query.DateTo.Month - query.DateFrom.Month
for(int i=0;i<months;i++){
//do stuff as below
var currentDate=query.DateFrom.AddMonths(i);
}
Requirements:
Calculate the number of months
between two dates: receiveDate and
dueDate.
Both optimistic and pessimistic
calculations are needed
Assumptions:
dueDate will always be the last day of the month.
I've already figured out the pessimistic calculation (meaning a single day overdue counts as a whole month:
if(receiveDate > dueDate)
receiveDate.Month - dueDate.Month + (receiveDate.Year - dueDate.Year) * 12;
Doing a search on the internet turned up several similar examples to confirm this.
Now my instincts tell me the optimistic calculation will just be the same minus one month but for some reason it just doesn't feel right. Am I on the right track or am I missing something?
You're right; if you're looking for the number of complete months between the two dates, subtracting 1 (assuming the receiveDate doesn't fall on the last day of the month, in which case you will have a remainder of 0 days either way) will get you your answer.
If you don't need to keep days of month in your calculus I think it's the way to go.
Your formula calculates the number of months between the first of the receivedDate's month to the first of the dueDate's month. As most time elements in TimeSpan are expressed as TotalXXX, it seems weird that they left out a TotalMonths and a TotalYears.
I think it's because there aren't a fixed number of days from month to month, so it's hard to know what makes most sense in terms of how to express the fractional remainder.
My formula is this...
int nMonthDiff_FirstToFirst = DateTime.Now.Month - testDate.Month + ((DateTime.Now.Year - testDate.Year)* 12);
double dMonthDiff = (double)nMonthDiff_FirstToFirst + (DateTime.Now - testDate.AddMonths(nMonthDiff_FirstToFirst)).TotalDays / (double)DateTime.DaysInMonth(DateTime.Now.Year, DateTime.Now.Month);
So what I'm doing is basically getting the month difference like you are (first of the month to first of the month) then I project my testDate into the future by the month difference. Then with that TimeSpan I get the TotalDays and divide that by the number of days in that month. Thus I'm representing the fractional portion in terms of remaining days of the month.
So if you were going May 5st 2012 -> June 3rd 2012, your formula would return 1 for month difference when a full month hasn't passed yet. In my formula the TotalDays of the projected date would yield a negative fractional number of days, and when added to the 'first to first' month difference, it would take it in as needed.
I am doing this extra check to get precise amount of months:
numberOfMonths = receiveDate.Month - dueDate.Month + (receiveDate.Year - dueDate.Year) * 12;
if (receiveDate.Day > dueDate.Day) numberOfMonths--;
int GetMonthsCount(DateTime dtStart, DateTime dtEnd)
{
return (int)Math.Round((dtEnd - dtStart).TotalMonths);
}