How to Get the Datetime For the Start of the Week

How can I get the DateTime for the start of the week?

Use an extension method. They're the answer to everything, you know! ;)

public static class DateTimeExtensions
{
    public static DateTime StartOfWeek(this DateTime dt, DayOfWeek startOfWeek)
    {
        int diff = (7 + (dt.DayOfWeek - startOfWeek)) % 7;
        return dt.AddDays(-1 * diff).Date;
    }
}

Which can be used as follows:

DateTime dt = DateTime.Now.StartOfWeek(DayOfWeek.Monday);
DateTime dt = DateTime.Now.StartOfWeek(DayOfWeek.Sunday);

Get the date of the start week of a given date c#

I would do something like this:

Parse the date from string to DateTime object (if required)

Add days current day of the week * -1 (turn it negative) + 1

string s = "2018-08-23 13:26";

DateTime dt = DateTime.ParseExact(s, "yyyy-MM-dd HH:mm", CultureInfo.InvariantCulture);

DateTime startOfWeek = dt.AddDays(((int)(dt.DayOfWeek) * -1) + 1);
Console.WriteLine(startOfWeek);

Here's a Fiddle

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Edit: If you are bothered about the sunday going to the next Monday,
then change the Sunday to remove 7 days from the current date like so;

string s = "2018-08-17 13:26";

DateTime dt = DateTime.ParseExact(s, "yyyy-MM-dd HH:mm", CultureInfo.InvariantCulture);

bool isSunday = dt.DayOfWeek == 0;
var dayOfweek = isSunday == false ? (int)dt.DayOfWeek : 7;

DateTime startOfWeek = dt.AddDays(((int)(dayOfweek) * -1) + 1);
Console.WriteLine(startOfWeek);

How to get start of or end of week in dart

You can get the weekday from the DateTime using https://api.dart.dev/stable/2.5.1/dart-core/DateTime/weekday.html and add/subtract this number from you date:

void main() {
  final date = DateTime.parse('2019-10-08 15:43:03.887');
  
  print('Date: $date');
  print('Start of week: ${getDate(date.subtract(Duration(days: date.weekday - 1)))}');
  print('End of week: ${getDate(date.add(Duration(days: DateTime.daysPerWeek - date.weekday)))}');
}

DateTime getDate(DateTime d) => DateTime(d.year, d.month, d.day);

UPDATE

Please read and upvote the answer from lrn. He knows a lot more about this stuff than me. :)

How to set start of the week as Sunday and end as Saturday in Dart?

Since DateTime.weekday counts from 1 (Monday) to 7 (Sunday):

date.subtract(Duration(days: date.weekday - 1))

will give us the date of the start of the week using what DateTime considers to be the first day of the week (Monday). To treat Sunday as the first day of the week instead, we can try to get the previous day from that:

date.subtract(Duration(days: date.weekday - 1)).subtract(const Duration(days: 1))

The above is algebraically equivalent to date - (weekday - 1) - 1, which can be simplified to just date - weekday. But that's not quite right because weekday ranges from [1, 7], and we never want to subtract 7 days. In that case, we want to subtract 0. The modulus operator handles that nicely, ultimately giving us:

DateTime findFirstDateOfTheWeek(DateTime date) {
  return date.subtract(Duration(days: date.weekday % DateTime.daysPerWeek));
}

To get the last day of the week where Saturday is treated as the last day of the week, you can just get the first day of the week and add 6 days:

DateTime findLastDateOfTheWeek(DateTime date) {
  return findFirstDateOfTheWeek(date).add(
    const Duration(days: DateTime.daysPerWeek - 1));
}

Get date of first Monday of the week?

This is what i use (probably not internationalised):

DateTime input = //...
int delta = DayOfWeek.Monday - input.DayOfWeek;
DateTime monday = input.AddDays(delta);

get the start and end timestamp of the current week in python

You can use datetime.replace():

from datetime import date, datetime, timedelta
today = datetime.now() # or .today()
start = (today - timedelta(days=today.weekday())).replace(hour=0, minute=0, second=0, microsecond=0)
end = (start + timedelta(days=4)).replace(hour=23, minute=59, second=0, microsecond=0)
print("Today: " + str(today))
print("Start: " + str(start))
print("End: " + str(end))

output

Today: 2021-07-08 22:56:19.277242
Start: 2021-07-05 00:00:00
End: 2021-07-09 23:59:00

Dart/Flutter get first DateTime of this week

If Sunday is your first day of week.

var d = DateTime.now();
var weekDay = d.weekday;
var firstDayOfWeek = d.subtract(Duration(days: weekDay));

According to Flutter docs weekDay returns

The day of the week [monday]..[sunday].

• In accordance with ISO 8601
• a week starts with Monday, which has the value 1.

if Monday is first day of week then

var firstDayOfWeek = d.subtract(Duration(days: weekDay - 1));

A simpler way of finding the start of the week for any start day

The number of days between START_DAY and now is now.weekday() - START_DAY. However, if today is before START_DAY (e.g. if today is Wednesday and START_DATE is 6 = Sunday), that number will be negative, and subtracting it from today would give you a date in the future.

Since you probably don't want that, you need the difference modulo 7:

week_start = now.date() - timedelta(days=(now.weekday() - START_DAY) % 7)

Examples:

>>> from datetime import datetime, timedelta
>>> now = datetime(2017, 2, 1)
>>> print(now.strftime("%d/%m/%y (%a)"))
01/02/17 (Wed)
>>> for START_DAY in range(7):
...     start_week = now.date() - timedelta(days=(now.weekday() - START_DAY) % 7)
...     print(start_week.strftime("%d/%m/%y (%a)"))
... 
30/01/17 (Mon)
31/01/17 (Tue)
01/02/17 (Wed)
26/01/17 (Thu)
27/01/17 (Fri)
28/01/17 (Sat)
29/01/17 (Sun)

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