My understanding is that DateTime.Today gives the time of midnight today in the local timezone.
Does DateTime.Today.AddDays(1) always give midnight tomorrow in the local timezone, or will it sometimes be off due to daylight savings?
i.e is .AddDays(1) == .AddHours(24), or is it sometimes different as it adjusts for daylight savings.
If .AddDays(1) == .AddHours(24), is there a way to get the time of midnight tomorrow local (which may be 23, 24 or 25 hours from now)?
The key element is to remember that DateTime is 'just' a container which holds so called ticks which are
"the number of 100-nanosecond intervals that have elapsed since 1/1/0001 12:00am". DateTime does not keep information which timezone the time it points to refers to.
When you call AddDays you are actually adding a constant TicksPerDay ticks:
public DateTime AddDays(double value) {
return Add(value, MillisPerDay);
}
private DateTime Add(double value, int scale) {
(...)
return AddTicks(millis * TicksPerMillisecond);
}
AddHours also results in ticks being added as well.
When you call AddHours(24) you are saying please add 24 hours to "YYYY-MM-DD HH:mm:SS" (15:00 -> 15:00 next day, independent of the timezone, or DST). You're not asking for the moment of time 24 hours later (24h later could be 14:00 or 16:00).
In order to answer "What time will it be 24h from now?" the timezone information is needed (see NodaTime below).
Similarly, AddHours(25) means: please add 1 day and 1 hour to "YYYY-MM-DD HH:mm:SS". With DateTime 2019-Oct-06 14:07 plus 25 hours is always 2019-Oct-07 15:07.
Please note that DateTime accounts for leap years (which requires only static rules).
Noda times allows you to 'pass' time i.e. add an amount of elapsed time. In other words it allows you to answer the question "What time will it be in n hours?"
In Date and time arithmetic we can read.
Time line arithmetic is pretty simple, except you might not always get what you expect when using ZonedDateTime, due to daylight saving transitions:
DateTimeZone london = DateTimeZoneProviders.Tzdb["Europe/London"]; // 12:45am on March 25th 2012 LocalDateTime local = new LocalDateTime(2012, 3, 25, 0, 45, 00); ZonedDateTime before = london.AtStrictly(local); ZonedDateTime after = before + Duration.FromMinutes(20);We start off with a local time of 12.45am. The time that we add is effectively "experienced" time - as if we'd simply waited twenty minutes. However, at 1am on that day, the clocks in the Europe/London time zone go forward by an hour - so we end up with a local time of 2:05am, not the 1:05am you might have expected.
The reverse effect can happen at the other daylight saving transition, when clocks go backward instead of forward: so twenty minutes after 1:45am could easily be 1:05am! So even though we expose the concept of "local time" in ZonedDateTime, any arithmetic performed on it is computed using the underlying time line.
My understanding is that
DateTime.Todaygives the time of midnight today in the local timezone.
That is mostly correct, but has an edge case. It will indeed give you the current date, using the local time zone in determining it, with the time set to 00:00:00.0000000. The resulting .Kind will be DateTimeKind.Local. What it does is equivalent to any of the following:
DateTime.Now.Date
DateTime.UtcNow.ToLocalTime().Date
TimeZoneInfo.ConvertTime(DateTime.UtcNow, TimeZoneInfo.Local).Date
// etc.
(See Difference between System.DateTime.Now and System.DateTime.Today for a complete list.)
Notice in all my examples, .Date is last. That property simply sets the time to zero. It keeps the .Kind, but it doesn't evaluate it in any way.
Where the edge case comes into play is if you are dealing with a time zone which has a transition at midnight - where the clocks go from 23:59 to 01:00. Since there is no 00:00, then the result from DateTime.Today will be a local time that doesn't exist. One such example is Santiago, Chile on 2019-09-08. Midnight doesn't exist there on that day.
Does
DateTime.Today.AddDays(1)always give midnight tomorrow in the local timezone, or will it sometimes be off due to daylight savings?i.e is
.AddDays(1) == .AddHours(24), or is it sometimes different as it adjusts for daylight savings.
Neither the AddDays or AddHours functions take time zone into account. They both simply add exactly 24 hours. Like the .Date property, these functions retain the .Kind, but they do not evaluate it in way.
If
.AddDays(1) == .AddHours(24), is there a way to get the time of midnight tomorrow local (which may be 23, 24 or 25 hours from now)?
Keeping in mind that midnight might not exist, as mentioned earlier, you simply need to adjust for that possibility. Here is an extension method to do that using only DateTime:
static DateTime AdjustToValidTime(this DateTime dt, TimeZoneInfo tz)
{
if (!tz.IsInvalidTime(dt))
{
// The time is already valid
return dt;
}
// Get the adjustment rule that applies
var rule = tz.GetAdjustmentRules()
.First(x => x.DateStart <= dt && x.DateEnd > dt);
// Get the transition gap. Often will be one hour, but not always
TimeSpan gap = rule.DaylightDelta;
// Add the gap to adjust to a valid time
return dt.Add(gap);
}
You could use it like this (for example):
DateTime tomorrow = DateTime.Today.AddDays(1).AdjustToValidTime(TimeZoneInfo.Local);
If you want to see the case of a day without midnight, you can use it like this:
TimeZoneInfo tz = TimeZoneInfo.FindSystemTimeZoneById("Pacific SA Standard Time");
DateTime today = new DateTime(2019, 9, 8);
DateTime adjusted = today.AdjustToValidTime(tz); // Will be 2019-09-08 01:00
Daylight saving is not adjusted automatically you need to adjust that from code and then you can use
DateTime.Today.AddDays(1)
which will always give you midnight time of the next day even in daylight saving.
To properly manage timezone and daylight saving use Use Noda time .net library
