# conversion time
###### tags: `C#`

var easternZoneId = "Singapore Standard Time";
//設定 TimeZoneInfo
TimeZoneInfo easternZone = TimeZoneInfo.FindSystemTimeZoneById(easternZoneId);
//換成 UTC
TimeZoneInfo.ConvertTimeToUtc(easternTime, easternZone);
//用系統的 timeinfo 轉換成 UTC
TimeZoneInfo.ConvertTimeToUtc(dateNow);

//將 utc 轉換成指定的 timezone
TimeZoneInfo.ConvertTimeFromUtc(timeUtc, easternZone);

各個 Id 名稱所對應的 timezone，可以從[這邊](https://stackoverflow.com/a/17300423/19304838)查
[Converting times between time zones 官方文件](https://learn.microsoft.com/en-us/dotnet/standard/datetime/converting-between-time-zones)

可能因為作業系統不同出現錯誤訊息 `System.IO.FileNotFoundException : Could not find file '/usr/share/zoneinfo/Singapore Standard Time'.`，這時就需要判斷作業系統給不同的設定
Linux 對應的 Id 可以[這裡查詢](https://en.wikipedia.org/wiki/List_of_tz_database_time_zones)

public static TimeZoneInfo GetSingaporeZone()
{
    if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
    {
        return TimeZoneInfo.FindSystemTimeZoneById("Singapore Standard Time");
    }
    if (RuntimeInformation.IsOSPlatform(OSPlatform.Linux))
    {
        return TimeZoneInfo.FindSystemTimeZoneById("Asia/Singapore");
    }
    if (RuntimeInformation.IsOSPlatform(OSPlatform.OSX))
    {
        return TimeZoneInfo.FindSystemTimeZoneById("Asia/Singapore");
    }

    return null;
}

[TimeZoneInfo in .NET Core when hosting on unix (nginx)](https://stackoverflow.com/a/55511180/19304838)