How can I change only the time in my DateTime variable "s"?
DateTime s = some datetime;
Best Answer
You can't change a DateTime value - it's immutable. However, you can change the variable to have a new value. The easiest way of doing that to change just the time is to create a TimeSpan with the relevant time, and use the DateTime.Date property:
DateTime s = ...;
TimeSpan ts = new TimeSpan(10, 30, 0);
s = s.Date + ts;
s will now be the same date, but at 10.30am.
Note that DateTime disregards daylight saving time transitions, representing "naive" Gregorian time in both directions (see Remarks section in the DateTime docs). The only exceptions are .Now and .Today: they retrieve current system time which reflects these events as they occur.
This is the kind of thing which motivated me to start the Noda Time project, which is now production-ready. Its ZonedDateTime type is made "aware" by linking it to a tz database entry.
// Save today's date.
var today = DateTime.Today;
// Calculate the age.
var age = today.Year - birthdate.Year;
// Go back to the year in which the person was born in case of a leap year
if (birthdate.Date > today.AddYears(-age)) age--;
However, this assumes you are looking for the western idea of the age and not using East Asian reckoning.
Best Answer
You can't change a DateTime value - it's immutable. However, you can change the variable to have a new value. The easiest way of doing that to change just the time is to create a TimeSpan with the relevant time, and use the DateTime.Date property:
s
will now be the same date, but at 10.30am.Note that
DateTime
disregards daylight saving time transitions, representing "naive" Gregorian time in both directions (see Remarks section in theDateTime
docs). The only exceptions are.Now
and.Today
: they retrieve current system time which reflects these events as they occur.This is the kind of thing which motivated me to start the Noda Time project, which is now production-ready. Its
ZonedDateTime
type is made "aware" by linking it to atz
database entry.