Question

Since silverlight doesn’t have complete TimeZoneInfo I will need to implement some of the functionality myself. For example, this following info comes from .NET TimeZoneInfo

ID: Central Standard Time
   Display Name:   (UTC-06:00) Central Time (US & Canada)
   Standard Name:                   Central Standard Time
   Daylight Name:                   Central Daylight Time   ***Has Daylight Saving Time***
   Offset from UTC:                      -6 hours, 0 minutes
   Number of adjustment rules:                          2
   Adjustment Rules:
      From 1/1/0001 12:00:00 AM to 12/31/2006 12:00:00 AM
      Delta: 01:00:00
      Begins at 2:00 AM on Sunday of week 1 of April
      Ends at 2:00 AM on Sunday of week 5 of October
      From 1/1/2007 12:00:00 AM to 12/31/9999 12:00:00 AM
      Delta: 01:00:00
      Begins at 2:00 AM on Sunday of week 2 of March
      Ends at 2:00 AM on Sunday of week 1 of November

My question is how do I implement such rule when I need to convert from UTC to local and back?

Should I take UTC, substract 6 hours and then check if it’s after 2am on Sunday of week 2 of March and then add 1hr?

Or should I take UTC, check if it’s after 2am and then substract 5?

Answer 1

The time that Daylight Saving Time takes effect is local to that timezone, not UTC. So you should take the UTC time, subtract 6 hours off, do the check and then add the hour back on if applicable.