User time zone

edit

With Blazor Server, the .NET code runs on the server, so TimeZoneInfo.Local is the time zone of the server, not the user's. TimeZoneService asks the browser. Register it with builder.Services.AddTimeZoneServices(), then inject it:

@inject TimeZoneService TimeZoneService

<button @onclick="ConvertToLocalTime">Show the local time</button>
<p>@localTime</p>

@code {
    private DateTimeOffset? localTime;

    private async Task ConvertToLocalTime()
    {
        var utcTime = DateTimeOffset.UtcNow;
        localTime = await TimeZoneService.GetLocalDateTimeAsync(utcTime);
    }
}

Method	Returns
`GetLocalDateTimeAsync(DateTimeOffset)`	The same instant, in the user's offset at that instant
`GetUtcDateTimeAsync(DateTime)`	The UTC instant of a local time of the user. A `DateTime` whose `Kind` is `Utc` is returned unchanged
GetOffsetAsync()	The user's offset from UTC now
`GetOffsetAsync(DateTimeOffset)`	The user's offset from UTC at the given instant
GetTimeZoneAsync()	The user's `TimeZoneInfo`, or `null` when the runtime does not know the identifier reported by the browser

Daylight saving time

The offset of a time zone changes during the year. Each conversion uses the offset at the instant being converted, not the current offset: converting a date in July and a date in January can give two different offsets.

GetUtcDateTimeAsync does not throw for a local time that does not exist (skipped when the clocks move forward) or that exists twice (when they move back): it picks one offset.

How the time zone is found

The service reads the IANA identifier of the browser time zone (such as Europe/Paris) once per scope and looks it up with TimeZoneInfo.TryFindSystemTimeZoneById. When the runtime does not know it, for instance without time zone data, GetTimeZoneAsync returns null and the offsets are asked to the browser for each instant instead.

The service calls JavaScript: call it from OnAfterRenderAsync or from an event handler, not during prerendering. To let the user type a date in their own time zone, pass the offset to InputDateTime.