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c#: How do I get the previous year-to-date and previous month-to-date for specific time periods?

Using c#, I want to compare the current week-to-date to the same period last week-to-date. For example, if today is Wednesday, and if the first day of the week is Sunday, then I want to compare totals for Sunday – Tuesday of this week against Sunday – Tuesday of last week. I’m not counting Wednesday because I don’t have a full day of data until midnight the same day.
The same applies to comparing this mtd to the same number of days last month, and last year. For example, if the current date is June 19th, I want to compare the data from May 1-18th of last month as well as January 1 – June 18th of last year against January 1 – June 18th of this year.
The variables I’m trying to use look like this:
//Current dates
DateTime currentDte = DateTime.Now;
DateTime beginWeek = DateTime.Now.StartOfWeek(DayOfWeek.Sunday);
DateTime beginMonth = new DateTime(DateTime.Now.Year, DateTime.Now.Month, 1);
DateTime beginYear = new DateTime(DateTime.Now.Year, 1, 1);
//Historical dates
DateTime lastWeekToDate = DateTime.Now.StartOfWeek(DayOfWeek.Sunday - (7 - (int)DateTime.Now.DayOfWeek));
DateTime lastMonthToDate =
DateTime lastYearToDate =
As you can see I figured out the current dates and can loop through them to get the wtd, mtd, and ytd data I need. And I managed to figure out how to get the last week-to-date I need.
But I don’t know how to get the lastMonthToDate and lastYearToDate dates I need. I’ve tried everything I can think of. I’ve read date documentation until my eyes hurt, and still I come up with goose eggs. Can anyone offer any suggestions?

If I understand your question correctly, you just need to use AddMonths and AddYears.
DateTime lastMonthToDate = beginMonth.AddMonths(-1);
DateTime lastYearToDate = beginYear.AddYears(-1);
EDIT
Based your comment - it looks like you want the to subtract months/years from the current date in which case it would be
DateTime lastMonthToDate = currentDte.AddMonths(-1);
DateTime lastYearToDate = currentDte.AddYears(-1);
If it's the time of day that's throwing you off, just use the date part DateTime.Now.Date.AddMonths(-1)

Based on the feedback I received from Zeph, I was able to get the beginning of last year using this code:
DateTime startDate = beginYear.AddYears(-1);
Now I'm able to tally all of the ytd data I need based on the interval dates. Thank you very much!

Julian Time stamp to Date Time

I am trying to convert to Julian Time Stamp to Date Time. I have the following microseconds time stamp 212302469304212709. As i understand i need to add these milliseconds to the beginning of Julian Calendar (January 1, 4713 B.C., 12:00 (noon)). So i have the following method:
private DateTime GetDateTime(string julianTimeStamp)
{
var julianMilliseconds = Convert.ToDouble(julianTimeStamp)/1000;
var beginningOfTimes = new DateTime(1, 1, 1, 0, 0, 0, 0);
var dateTime = beginningOfTimes.AddMilliseconds(julianMilliseconds).AddYears(-4713).AddMonths(-1).AddDays(-1).AddHours(-12);
return dateTime;
}
Assume i pass 212302469304212709 string as the parameter. The expected result should be 2015/07(July)/01 00:08:24.212. Based on my method, i have almost the same result, but day is not 1, it is 6. Same problem for different time stamps i tested.
Could any one tell me what i am doing wrong? Thanks in advance.
Edited:
This is the exact date time i expect to receive: 2015(year) 7(month) 1(day) 0(hour) 8(minute) 24(second) 212(millisecond) 709(microsecond)

The given timestamp 212,302,469,304,212,709 μs when converted to days (just divide by 86,400,000,000) gives 2457204.505836 days (to six decimal places, which is the best I can do without a lot of extra trouble). Using the Multi Year Computer Interactive Almanac (MICA) written by the United States Naval Observatory, and putting in the free form date 2015(year) 7(month) 1(day) 0(hour) 8(minute) 24(second) 212(millisecond) 709(microsecond), the program calculates exactly the same day count (to six decimal places), proving the time stamp is an accurate Julian date.
One problem with the OP's calculation is trying to use the DateTime class before the earliest supported date, as pointed out by another poster. Also, the OP didn't say if 1 July 2015 was in the Julian or Gregorian calendar, but the MICA calculation proves it is in the Gregorian calendar. Since the OP is working in the Gregorian calendar, the epoch of Julian dates should be stated in the Gregorian proleptic calendar: Noon Universal Time, November 24, 4714 BC. The oft-quoted date January 1, 4713 BC is a proleptic Julian calendar date.
"Proleptic" means a date has been found by beginning at a modern date, who's calendar date is known with absolute certainty, and applying the rules of the chosen calendar backward until the desired date is reached, even though the desired date is before the chosen calendar was invented.

DateTime uses Gregorian calendar, so when you substract years, months and so on you are doing it with that calendar, not the Julian.
Unfortunately DateTime does not support dates before year 1. You can check the library in this post, maybe it helps you.

DateTime.Today , beginning or end of the day?

I am using DateTime.Today.
Now I'm not sure if the date is from the beginning of the day or the end of the day.
This is what DateTime.Today returns : {11-3-2014 0:00:00}

MSDN states the following: "An object that is set to today's date, with the time component set to 00:00:00."
This means that a DateTime object is created with today's date at the absolute start of the day hence 00:00:00.
You can check if it is the start of the day by using the AddHour() method of the DateTime class.
DateTime d = DateTime.Today;
//AddHours, AddMinutes or AddSeconds
d = d.AddHours(1);
if (d.Date != DateTime.Today.Date)
{
//Not the same day
}
If d.date should be different the date was initialised at a different time (eg. 23:00:01).
http://msdn.microsoft.com/en-us/library/system.datetime.today(v=vs.110).aspx

0:00:00 is the start of the day and 23:59:59 is the end of day.
You can also confirm through this 24-hour clock
In the 24-hour time notation, the day begins at midnight, 00:00, and the last minute of the day begins at 23:59. Where convenient, the
notation 24:00 may also be used to refer to midnight at the end of a
given date[5] – that is, 24:00 of one day is the same time as 00:00 of
the following day.
On a side note:-
If you want to know the time then use .Now because that includes the 10:32:32 or whatever time; however .Today is the date-part only (at 00:00:00 on that day ie the begining of the day). So you can say that .Today is essentially the same as .Now.Date

Thats the beginning of the day, the end of the day would be:
{11-3-2014 23:59:59}
And remember, the only stupid question is the one you don't ask :)

DateTime.Today returns the current DateTime value, without the Time part.
Which means, it is the the first possible DateTime value for the current day.

Think of it like this:
The last moment in a day can be 23:59 or theoretically any amount of nanseconds before the next day. the next day then starts at 00:00:00 counting upwards.
So 11-3-2014 0:00:00 marks the beginning of a day. Either the earliest possible moment, or no time at all, if you want to treat 0:00:00 as a default value.

Iterating through a TimeSpan and running an action every month (or arbitrary unit of time)

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);
}

function which divides time period by periodicity

I need help to creating a function that can divide a period of time into regular periodicity of year.
For example: I have a period from 11/10/2011 to 08/07/2012 divided on regular semester. I want to get this result in a list:
1- 11/10/2011 - 12/31/2011
2- 01/01/2012 - 06/30/2012
3- 07/01/2012 - 08/07/2012

As you said, it is little bit complex but not impossible. With small thought you might get this easily. I've implemented code but to give your brain some work I'm not posting the code but giving you pseudo code below.
I thought of following way. First you need to have a duration through which you want to divide a year, for ex: 6 months.
Take the start date and create a new date by using DateTime constructor (year, month, day) and pass the year in the start date as year and month and day as 1 as below:
new DateTime(startdate.Year, 1, 1);
This will give you start date of that year. Then add duration days/months to that date to get the next periodicity. If your start date is less than this new date then add once again duration to that new date till you get the date greater than start date.
With the above logic you can form the periods you want till the end date. Of course you have to check whether your end date as well against this periodicity. There are many other conditions you need to check to get the proper system to give these periods for any given start and end dates.