How do I convert a string+offset value to a DateTimeOffset without adding-then-subtracting the offset?

Our app interacts with an API that returns local datetime strings in the format yyyyMMdd HH:mm with a secondary number field indicating the UTC offset in hours.

We’re successfully handling this with the following code.

DateTimeOffset ParseWeirdDateTime(string dateTimeString, double utcOffset)
{
    DateTimeOffset dateTimeOffset = DateTimeOffset.ParseExact(
        dateTimeString,
        "yyyyMMdd HH:mm",
        CultureInfo.InvariantCulture,
        DateTimeStyles.AssumeUniversal
    );

    return dateTimeOffset.AddHours(-utcOffset).ToOffset(TimeSpan.FromHours(utcOffset));
}

string localDateTimeString = "20231108 14:30";
double utcOffset = -4;
DateTimeOffset dateTimeParsed = ParseWeirdDateTime(localDateTimeString, utcOffset);
// comes out to 11/8/2023 2:30:00 PM -04:00

My question is, is there a better way to handle this? Though this works, it feels clunky to parse to a UTC DateTimeOffset, only to apply the inverse of the offset and finally convert to the offset.

>Solution :

• You say your yyyyMMdd HH:mm input strings represent a "local" (or rather: non-UTC) instant, but…
  • You’re using DateTimeOffset.ParseExact even though the input string does not represent a Date+Time+Offset tuple: the string only represents a Date+Time+Undefined-Offset tuple.
    • So use DateTime.ParseExact instead, and ensure it has DateTimeKind.Unspecified (because no offset data is specified at-this-point).
    • (btw, never use DateTimeKind.Local: it specifically refers to the local computer’s time, which is almost always useless in business applications because it isn’t invariant like real UTC is: consider local timezones, political timezone changes, daylight savings, machine misconfiguration, etc).
  • Also, you’re incorrectly using DateTimeStyles.AssumeUniversal, which doesn’t apply here because the string you’re passing-in does not represent a UTC value. As the documentation states "If no time zone is specified in the parsed string, the string is assumed to denote a UTC."

I’d do it like so:

• As the dateTimeString value only represents a date+time value without an offset I use DateTime.ParseExact instead of DateTimeOffset.ParseExact.
• Use DateTimeStyles.None as AssumeUniversal is incorrect here.
• Then I adjust the DateTime value to a DateTimeOffset via the ctor.

/// <summary><see href="https://stackoverflow.com/a/77449580/159145" /></summary>
static DateTimeOffset ParseWeirdDateTime( String dateTimeString, Double utcOffsetInHours /* Always specify units in names when using dimensionless types */ )
{
    DateTime instantInTimeWithUndefinedOffset = DateTime.ParseExact(
        s       : dateTimeString,
        format  : "yyyyMMdd HH:mm",
        provider: CultureInfo.InvariantCulture,
        style   : DateTimeStyles.None
    );
    
    Debug.Assert( instantInTimeWithUndefinedOffset.Kind == DateTimeKind.Unspecified );

    return new DateTimeOffset( instantInTimeWithUndefinedOffset, offset: TimeSpan.FromHours( utcOffsetInHours ) );
}