How to convert DateTime in Specific timezone?

Question

I find it hard to understand how UTC works.

I have to do the following but I'm still confused if I'd get the right result.

Objectives:

1. Ensure all saved dates in Database are in UTC format
2. Update DefaultTimezone is in Manila time
3. Ensure all returned dates are in Manila Time

So the code is:

public ConvertDate(DateTime? dateTime)
{
    if (dateTime != null)
    {
        Value = (DateTime)dateTime;
        TimeZone = GetFromConfig.DefaultTimeZone(); 
    }
}


public ConvertDate(DateTime? dateTime, int GMTTimeZone)
{
    if (dateTime != null)
    {
        Value = (DateTime)dateTime;
        TimeZone = GMTTimeZone;
    }
}


public int TimeZone
{
    get { return m_TimeZone; }
    set { m_TimeZone = value; }
}


DateTime m_Value;
public DateTime Value
{
    get { return m_Value; }
    set 
    { 
        m_Value = value;
        DateTime converted = m_Value.ToUniversalTime().ToLocalTime();
    }
}

Sample usage:

DateTime SampleInputFromUser = new DateTime(2012, 1, 22);
ConvertDate newConversion = new ConvertDate(SampleInputFromUser, 21);
DateTime answer = newConversion.Value;

Now I get confused for 'TimeZone'. I don't know how to use it to get the objectives.
Hope you understand my question and have the idea to get the objectives done.

Edit

According to @raveturned answer, I get this following code:

***Added in ConvertDate method

TimeZoneInfo timeInfo = TimeZoneInfo.FindSystemTimeZoneById(GetFromConfig.ManilaTimeZoneKey());
ManilaTime = TimeZoneInfo.ConvertTime(dateTime.Value, TimeZoneInfo.Local, timeInfo).ToUniversalTime();

**New Property

DateTime _ManilaTime;
public DateTime ManilaTime
{
    get { return _ManilaTime; }
    set { _ManilaTime = value; }
}

Answer 1

The .NET framework already has classes and methods available to convert DateTimes between different time zones. Have a look at the ConvertTime methods of the TimeZoneInfo class.

Edit: When you get the time to put into the database, assuming it was created with correct time zone information you can easily convert to UTC:

DateTime utcTime = inputDateTime.ToUniversalTime();

Get timeInfo as done in the question edit:

TimeZoneInfo timeInfo = TimeZoneInfo.FindSystemTimeZoneById(GetFromConfig.ManilaTimeZoneKey());

When you send the database time to user, convert it to the correct timezone using timeInfo.

DateTime userTime = TimeZoneInfo.ConvertTimeFromUtc(dbDateTime, timeInfo);

Personally I'd try and keep this logic separate from the propery get/set methods.