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Subtracting an unknown interval from a datetime

I am setting up a system to gather data from a database based on a user inputted start date and end date. The system will gather data averaged over an interval(1 hour, 6 hours, or one day for example). If the user does not input a start or end date I would like the program to set the start date to the current time minus the interval.
I currently have the user inputting the interval in the following format.
1m = 1 minute
1h = 1 hour
12h = 12 hours
3d = 3 days
So these values are not formatted like datetime. I could take the current datetime and subtract it by either minutes, hours, or days depending on the value appended (splitting on the number), but this would mean many if statements. What I would really like is a method to subtract a datetime by an arbitrary value Does anyone have a better solution?

Instead of providing predefined time intervals (that are implemented e. g. via a separate type/enum), it is much easier to let the user freely specify a TimeSpan.
This has two advantages:
The user is not restricted to predefined intervals
You can subtract the TimeSpan directly from your DateTime.Now
If restriction to limited intervals is a requirement, you can implement this in the view/window. But still this should be a TimeSpan.

How can I determine the last day of the month [duplicate]

This question already has answers here:
Last day of the month in .NET
(5 answers)
Closed 7 years ago.
How can I determine the last day of the month, for the tested month, when the user enters a date into textbox that is further from the available last day for that month? Also how do I feed that back to the user.
When the user enters the date, for example, of April 31, 2015, the date should automatically change to April 30, 2015.
I would like to try doing this using c#

3rd EDIT: The below will get you close to what you're asking for but really just make use of a datepicker control it will help prevent the majority of invalid dates and save you a good amount of development time
2nd EDIT: So TryParse is beneficial because if the date is valid, continue and your done. If you have an invalid date, which TryParse will tell you, then you can compare the value that was input for the day value and see how close it is to the end of the month, i.e. take 31 - 30 = 1, 1 number off so they probably meant 30
EDIT: To answer your question about finding an invalid date check this out: Validate a DateTime in C#
It shows how to detect an invalid date.
To answer your question about how to "guess" which validate date they are closest to I'd say it depends on how you have your date entered. If you do something like have them enter an int value for the day value I'd check to see what the highest day value is for the selected month and then see which one they are closest to (this is just a math operation at this point).
I think this will work
DateTime lastday = new DateTime(DateTime.Today.Year, DateTime.Today.Month, DateTime.DaysInMonth(DateTime.Today.Year, DateTime.Today.Month));

This is an easy way:
//first day of next month
var dt = new DateTime(2015,4,1);
var lastDayOfMonth = dt.AddDays(-1).Day;
As far as delivering a message back to the user, it depends on the environment, technology and preferred approach.

C# - calculate if a date is six months older

I am trying to calculate if the given specified date is at least six months old. I am doing this:
if(DateTime.Now.AddMonths(-6)>date)
{
//Do something
}
Is this correct?
Some people say that this approach is wrong and will not give accurate results. Is the above is correct?

"6 months" is not a precise amount of time. It depends on the length of the months. In particular, you may well get different results from your calculation compared with date.AddMonths(6) < DateTime.Now. (Consider adding 6 months from August 30th vs taking away 6 months from February 28th... You may be okay, but you need to think about this carefully.)
You need to consider a few things carefully:
You're currently using DateTime.Now instead of DateTime.Today; how do you want the current time of day to affect things?
Is the "kind" of date UTC, unspecified or local? It makes a difference - DateTime is confusing, unfortunately.
How do you want to handle situations like the ones in the first paragraph?
Ultimately, if people are telling you it will not give accurate results, you should ask them for specific examples - you need to get a wealth of input data and desired results, add automated tests for them, and get them to pass. Then if anyone claims your code isn't working correctly, you should be able to challenge them to create another test case which fails, and justify their decision.

If you are only concerned about the date and not the time, use DateTime.Now.Date instead. Apart from that, I do not see any problems with the code you already have.

Strategy for Incomplete Dates

Working on an application where we would like the user to be able to enter incomplete dates.
In some cases there will only be a year - say 1854, or there might be a year and a month, for example March 1983, or there may be a complete date - 11 June 2001.
We'd like a single 'date' attribute/column - and to be able to sort on date.
Any suggestions?

Store the date as an integer -- yyyymmdd.
You can then zero out any month or day component that has not been entered
Year only: 1954 => 19540000
Year & Month: April 2004 => 20040400
January 1st, 2011 => 20110101
Of course I am assuming that you do not need to store any time of day information.
You could then create a struct to encapsulate this logic with useful properties indicating which level of granularity has been set, the relevant System.DateTime, etc
Edit: sorting should then work nicely as well

I can't think of a good way of using a single date field.
A problem you would get if you used January as the default month and 1 as the default day like others have suggested is, what happens when they actually pick January? How would you track if it's a selected January or a defaulted January.
I think you're going to have to store a mask along with the date.
You would only need a bit per part of the date, which would only be 6 bits of data.
M|D|Y|H|Min|S
Month Only 1|0|0|0|0|0 = 32
Year Only 0|0|1|0|0|0 = 8
Month+Year 1|0|1|0|0|0 = 40
AllButMinSec 1|1|1|1|0|0 = 60
You could put this into a Flag Enum to make it easier to use in code.

Well, you could do it via a single column and field that says 'IsDateComplete'.
If you only have the date field, then you'll need to encode the "incompleteness" in the date format itself, such that if the date is, say, < 1900, it's considered "Incomplete".
Personally, I'd go with an field on the side, that marks it as such. Easier to follow, easier to make decisions on, and allows for any dates.
It goes without saying, perhaps, that you can just create a date from DateTime.MinValue and then set what you "know".
Of course, my approach doesn't allow you to "know" what you don't know. (That is, you don't know that they've set the month). You could perhaps use a date-format specifier to mask that, and store it alongside as well, but it's potentially getting cumbersome.
Anyway, some thoughts for you.

One option is to use January as the default month, 1 as the default day, and 1900 or something like that as the default year. Incomplete dates would get padded out with those defaults, and incomplete dates would sort before complete ones in the same year.
Another, slightly more complex option is to use -1 for default day and year, and -1, 'NoMonth', or some such as the default month. Pad incomplete dates as above. This may make sorting a little hard depending on how you do it, but it gives you a way of telling which parts of the date are valid.

I know you'd rather have 1 column but, Instead of a single column one can always have a separate column for day, month and year. Not very difficult to do queries against, and it allways any of the components to be null.
Smehow encoding these states in the datetime itself will be harder to query.

What I did when last solving this problem, was to create a custom date type that kept track of which date parts was actually set and provided conversions to and from a DateTime. For storing in database i used one date field and then one boolean/bit to keep track of which date components that were actually set by the user.

Building a Datetime Object for SQL database insert

I'm working on a small web form that requires the user to input (among other things), the scheduled backup time of whatever server they're adding to the system. The problem is, I'm struggling to find out the best way to take the user input and build a DateTime object (which is what the database requires).
I only really care about the Day of Week, Time of Day (12 or 24 hour clock).
I thought about just creating an empty DateTime object and then just adding my input values from the user, but you can only get, not set, the day of week, time of day, etc.
I've been looking at the Calender asp control, which would work for the day of the week selection, but I can't seem to find any support of time of day.
Thanks.

I don't think you want to use a DateTime for a recurring event such as a backup. A DateTime is useful for storing a particular date and time, but not a "template" for a recurring event. Instead I'd use separate columns to store the day of week value (0-6) and time of date (minutes after midnight) for the event.

If you going to use datepicker here is one great sample for adding JQuery date picker using C#. That helped me including in my project evrn if I did know anything abaut JQuery and java sripts at all.

DateTime is a immutable value type. You cannot set anything on it.
Assumed that you stick with DateTime on the DB and you don't want to use a DateTimePicker control.
You have to specify how the day of week and the time should be represented in the DateTime. You can start with DateTime.MinValue, the 1.1.0001, 12:00 at midnight, and add the day of week and the time. unfortunately, a regular DateTime field in a SqlServer 2005 is not able to store this date. So lets move it to the year 2000. The 1.1.2000 was a Saturday. You could calculate the DateTime like this:
int dayOfWeek; // 0 = mon, 6 = son
DateTime time;
DateTime scheduleTime = new DateTime(2000, 1, (dayOfWeek + 2) % 6 + 1)
+ time.TimeOfDay;
But honestly, I wouldn't do it. It smells. I just answered your question. Listen to tvanfosson. He said everything that needs to be said.