Getting the first and last day of a month using a given DateTime object

Question

I want to get the first day and last day of the month where a given date lies in  The date comes from a value in a UI field   If I m using a time picker I could say  var maxDay   dtpAttendance MaxDate Day    But I m trying to get it from a DateTime object  So if I have this     DateTime dt   DateTime today    How to get first day and last day of the month from dt

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Here you can add one month for the first day of current month than delete 1 day from that day   DateTime now   DateTime Now  var startDate   new DateTime now Year  now Month  1   var endDate   startDate AddMonths 1  AddDays -1

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This is more a long comment on  Sergey and  Steffen s answers   Having written similar code myself in the past I decided to check what was most performant while remembering that clarity is important too  Result Here is an example test run result for 10 million iterations  2257 ms for FirstDayOfMonth AddMethod   2406 ms for FirstDayOfMonth NewMethod   6342 ms for LastDayOfMonth AddMethod   4037 ms for LastDayOfMonth AddMethodWithDaysInMonth   4160 ms for LastDayOfMonth NewMethod   4212 ms for LastDayOfMonth NewMethodWithReuseOfExtMethod   2491 ms for LastDayOfMonth SpecialCase    Code I used LINQPad 4  in C  Program mode  to run the tests with compiler optimization turned on  Here is the tested code factored as Extension methods for clarity and convenience  public static class DateTimeDayOfMonthExtensions       public static DateTime FirstDayOfMonth AddMethod this DateTime value                return value Date AddDays 1 - value Day                  public static DateTime FirstDayOfMonth NewMethod this DateTime value                return new DateTime value Year  value Month  1                  public static DateTime LastDayOfMonth AddMethod this DateTime value                return value FirstDayOfMonth AddMethod   AddMonths 1  AddDays -1                  public static DateTime LastDayOfMonth AddMethodWithDaysInMonth this DateTime value                return value Date AddDays DateTime DaysInMonth value Year  value Month  - value Day                  public static DateTime LastDayOfMonth SpecialCase this DateTime value                return value AddDays DateTime DaysInMonth value Year  value Month  - 1                  public static int DaysInMonth this DateTime value                return DateTime DaysInMonth value Year  value Month                  public static DateTime LastDayOfMonth NewMethod this DateTime value                return new DateTime value Year  value Month  DateTime DaysInMonth value Year  value Month               public static DateTime LastDayOfMonth NewMethodWithReuseOfExtMethod this DateTime value                return new DateTime value Year  value Month  value DaysInMonth              void Main         Random rnd   new Random        DateTime   sampleData   new DateTime 10000000            for int i   0  i  lt  sampleData Length  i              sampleData i    new DateTime 1970  1  1  AddDays rnd Next 0  365   50                   GC Collect        System Diagnostics Stopwatch sw   System Diagnostics Stopwatch StartNew        for int i   0  i  lt  sampleData Length  i              DateTime test   sampleData i  FirstDayOfMonth AddMethod              string Format  quot  0  ms for FirstDayOfMonth AddMethod   quot   sw ElapsedMilliseconds  Dump             GC Collect        sw Restart        for int i   0  i  lt  sampleData Length  i              DateTime test   sampleData i  FirstDayOfMonth NewMethod              string Format  quot  0  ms for FirstDayOfMonth NewMethod   quot   sw ElapsedMilliseconds  Dump             GC Collect        sw Restart        for int i   0  i  lt  sampleData Length  i              DateTime test   sampleData i  LastDayOfMonth AddMethod              string Format  quot  0  ms for LastDayOfMonth AddMethod   quot   sw ElapsedMilliseconds  Dump         GC Collect        sw Restart        for int i   0  i  lt  sampleData Length  i              DateTime test   sampleData i  LastDayOfMonth AddMethodWithDaysInMonth              string Format  quot  0  ms for LastDayOfMonth AddMethodWithDaysInMonth   quot   sw ElapsedMilliseconds  Dump         GC Collect        sw Restart        for int i   0  i  lt  sampleData Length  i              DateTime test   sampleData i  LastDayOfMonth NewMethod              string Format  quot  0  ms for LastDayOfMonth NewMethod   quot   sw ElapsedMilliseconds  Dump         GC Collect        sw Restart        for int i   0  i  lt  sampleData Length  i              DateTime test   sampleData i  LastDayOfMonth NewMethodWithReuseOfExtMethod              string Format  quot  0  ms for LastDayOfMonth NewMethodWithReuseOfExtMethod   quot   sw ElapsedMilliseconds  Dump         for int i   0  i  lt  sampleData Length  i              sampleData i    sampleData i  FirstDayOfMonth AddMethod                   GC Collect        sw Restart        for int i   0  i  lt  sampleData Length  i              DateTime test   sampleData i  LastDayOfMonth SpecialCase              string Format  quot  0  ms for LastDayOfMonth SpecialCase   quot   sw ElapsedMilliseconds  Dump            Analysis I was surprised by some of these results  Although there is not much in it the FirstDayOfMonth AddMethod was slightly faster than FirstDayOfMonth NewMethod in most runs of the test   However  I think the latter has a slightly clearer intent and so I have a preference for that  LastDayOfMonth AddMethod was a clear loser against LastDayOfMonth AddMethodWithDaysInMonth  LastDayOfMonth NewMethod and LastDayOfMonth NewMethodWithReuseOfExtMethod  Between the fastest three there is nothing much in it and so it comes down to your personal preference  I choose the clarity of LastDayOfMonth NewMethodWithReuseOfExtMethod with its reuse of another useful extension method   IMHO its intent is clearer and I am willing to accept the small performance cost  LastDayOfMonth SpecialCase assumes you are providing the first of the month in the special case where you may have already calculated that date and it uses the add method with DateTime DaysInMonth to get the result   This is faster than the other versions  as you would expect  but unless you are in a desperate need for speed I don t see the point of having this special case in your arsenal  Conclusion Here is an extension method class with my choices and in general agreement with  Steffen I believe  public static class DateTimeDayOfMonthExtensions       public static DateTime FirstDayOfMonth this DateTime value                return new DateTime value Year  value Month  1                  public static int DaysInMonth this DateTime value                return DateTime DaysInMonth value Year  value Month                  public static DateTime LastDayOfMonth this DateTime value                return new DateTime value Year  value Month  value DaysInMonth              If you have got this far  thank you for time  Its been fun        Please comment if you have any other suggestions for these algorithms

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DateTime dCalcDate   DateTime Now  var startDate   new DateTime Convert ToInt32 Year   Convert ToInt32 Month   1   var endDate   new DateTime Convert ToInt32 Year   Convert ToInt32 Month   DateTime DaysInMonth  Convert ToInt32 Year    Convert ToInt32 Month

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You can try this for get current month first day  DateTime Now AddDays - DateTime Now Day-1    and assign it a value  Like this  dateEndEdit EditValue   DateTime Now  dateStartEdit EditValue   DateTime Now AddDays - DateTime Now Day-1

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Getting month range with  Net API  just another way    DateTime date       var firstDayOfMonth   new DateTime date Year  date Month  1   var lastDayOfMonth   new DateTime date Year  date Month  DateTime DaysInMonth date Year  date Month

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Give this a try  It basically calculates the number of days that has passed on DateTime Now  then subtracts one from that and uses the new value to find the first of the current month  From there it uses that DateTime and uses  AddMonths -1  to get the first of the previous month   Getting the last day of last month does basically the same thing except it adds one to number of days in the month and subtracts that value from DateTime Now AddDays  giving you the last day of the previous month   int NumberofDays   DateTime Now Day  int FirstDay   NumberofDays - 1  int LastDay   NumberofDays   1  DateTime FirstofThisMonth   DateTime Now AddDays -FirstDay   DateTime LastDayOfLastMonth   DateTime Now AddDays -LastDay   DateTime CheckLastMonth   FirstofThisMonth AddMonths -1

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For Persian culture   PersianCalendar pc   new PersianCalendar                 var today   pc GetDayOfMonth DateTime Now   var firstDayOfMonth   pc GetDayOfMonth DateTime Now AddDays - today-1     var lastDayOfMonth   pc GetDayOfMonth DateTime Now AddMonths 1  AddDays -today                Console WriteLine  First day    firstDayOfMonth   Console WriteLine  Last day     lastDayOfMonth

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DateTime dCalcDate   DateTime Now  dtpFromEffDate Value   new DateTime dCalcDate Year  dCalcDate Month  1   dptToEffDate Value   new DateTime dCalcDate Year  dCalcDate Month  DateTime DaysInMonth dCalcDate Year  dCalcDate Month

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easy way to do it  Begin   new DateTime DateTime Now Year  DateTime Now Month 1  ToShortDateString    End   new DataFim Text   new DateTime DateTime Now Year  DateTime Now Month  DateTime DaysInMonth DateTime Now Year  DateTime Now Month   ToShortDateString

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The accepted answer here does not take into account the Kind of the DateTime instance  For example if your original DateTime instance was a UTC Kind then by making a new DateTime instance you will be making an Unknown Kind instance which will then be treated as local time based on server settings  Therefore the more proper way to get the first and last date of the month would be this   var now   DateTime UtcNow  var first   now Date AddDays - now Date Day - 1    var last   first AddMonths 1  AddTicks -1     This way the original Kind of the DateTime instance is preserved

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I used this in my script works for me  but I needed a full date without the need of trimming it to only the date and no time   public DateTime GetLastDayOfTheMonth         int daysFromNow   DateTime DaysInMonth DateTime Now Year  DateTime Now Month  -  int DateTime Now Day      return DateTime Now AddDays daysFromNow

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If you only care about the date    var firstDay   new DateTime date Year  date Month  1  0  0  0  date Kind   var lastDay   new DateTime date Year  date Month  1  0  0  0  date Kind  AddMonths 1  AddDays -1     If you want to preserve time    var firstDay   new DateTime date Year  date Month  1  date Hour  date Minute  date Second  date Kind   var lastDay   new DateTime date Year  date Month  1  date Hour  date Minute  date Second  date Kind  AddMonths 1  AddDays -1

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Last day of month  is actually  First day of  next  month  minus 1   So here s what I use  no need for  DaysInMonth  method   public static DateTime FirstDayOfMonth this DateTime value        return new DateTime value Year  value Month  1      public static DateTime LastDayOfMonth this DateTime value        return value FirstDayOfMonth            AddMonths 1           AddMinutes -1       NOTE  The reason I use AddMinutes -1   not AddDays -1  here is because usually you need these date functions for reporting for some date-period  and when you build a report for a period  the  end date  should actually be something like Oct 31 2015 23 59 59 so your report works correctly - including all the data from last day of month   I e  you actually get the  last moment of the month  here  Not Last day   OK  I m going to shut up now

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You can do it  DateTime dt   DateTime Now   DateTime firstDayOfMonth   new DateTime dt Year  date Month  1   DateTime lastDayOfMonth   firstDayOfMonth AddMonths 1  AddDays -1

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Try this one   string strDate   DateTime Now ToString  MM 01 yyyy

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DateTime structure stores only one value  not range of values  MinValue and MaxValue are static fields  which hold range of possible values for instances of DateTime structure  These fields are static and do not relate to particular instance of DateTime  They relate to DateTime type itself   Suggested reading  static  C  Reference   UPDATE  Getting month range   DateTime date       var firstDayOfMonth   new DateTime date Year  date Month  1   var lastDayOfMonth   firstDayOfMonth AddMonths 1  AddDays -1

[c#] Getting the first and last day of a month, using a given DateTime object

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