convert a timer to different time zone - asp.net

Question

how to convert a time value which display a duration, to another time value to another time zone please
I'm doing an auction website, i display the duration of each time, and how to ensure that if someone is in another country, it can show the duration, in his own time zone duration please
Protected Sub Timer1_Tick(ByVal sender As Object, ByVal e As EventArgs)
'--this method will fetch the end date from repeater item label and update the duration
UpdateDuration()
End Sub
Private Sub UpdateDuration()
For Each item As DataListItem In DataList1.Items
'--calculate the duration
Dim duration As TimeSpan = Convert.ToDateTime(TryCast(item.FindControl("lblEndDate"), Label).Text) - DateTime.Now
Dim label2 = TryCast(item.FindControl("lblDuration"), Label)
'--Here is the important part: if any duration is expired then bind the grid again
If (duration.Ticks < 0) Then
'Grab your DateTime object (checking that it exists)'
'Now you can output your time as you please (excluding any decimal points)'
BindItems()
Exit For
Else
'-- showing end date as last days has not arrived
TryCast(item.FindControl("lblDuration"), Label).Text = New DateTime(duration.Ticks).ToString("dd:HH:mm:ss")
label2.Text = String.Format("{0:dd\:hh\:mm\:ss}", duration)
label2.Text = Replace(label2.Text, ":", " Days ", , 1)
label2.Text = Replace(label2.Text, ":", " Hours ", , 1)
label2.Text = Replace(label2.Text, ":", " Mins ", , 1)
End If
Next
End Sub

Answer 1

This following line in javascript can give you the timezoneoffset of client machines time zone with respect to UTC (For further information visit this link):
var offset = new Date().getTimezoneOffset();
Once you know the offset you can make use of the following C# code to get the UTC time:
DateTime UtcNow = DateTime.UtcNow;
You can save the offset in a hidden field at client and once on server store it in a session for each user. Based on the sign of the offset you will need to decide whether to add or subtract minutes from UTC time. If the value is negative say -330 then add 330 mins to UTC datetime, and if value is positive say 20 then subtract 20 mins from UTC datetime. e.g.
DateTime clientTime=UtcNow.AddMinutes(20)
The above instructions will help you achieve what you desire. But I would also like to give you a piece of advise concerning security. No decision should be taken based on the data that you derive from the client as this data may have been tempered. Timezone offset that you fetch from the client should only be used to display time on their browser, but it should not affect the fact that your auction would run say 1hr from 11 AM CST. Hope you understand the risk here.
Hope this helps. Please ask for clarification if required, specially if you are not clear about what could be the risk.