diff --git a/calendar/base/public/calIDateTime.idl b/calendar/base/public/calIDateTime.idl
index ffcb4e3de5d4..036add2791d9 100644
--- a/calendar/base/public/calIDateTime.idl
+++ b/calendar/base/public/calIDateTime.idl
@@ -59,16 +59,18 @@ interface calIDateTime : nsISupports
 
   // this thing's base PRTime value, either as UTC or as timezoneless-
   // local.  Setting this will explode the PRTime value to year/etc
-  // based on the current value of timezoneOffset.
+  // based on the current value of timezone.  PRTime is milliseconds
+  // since the epoch, in UTC.
   attribute PRTime nativeTime;
 
+  //
   // Year, fully exploded (e.g. "1989", "2004")
   attribute short year;
 
   // Month, 0-11
   attribute short month;
 
-  // Day, 1 through whatever XXX - does libical want 1.. or 0..?
+  // Day, 1-[28,29,31]
   attribute short day;
 
   // Hour, 0-23
@@ -80,22 +82,19 @@ interface calIDateTime : nsISupports
   // Second, 0-59
   attribute short second;
 
-  // whether this thing indicates a time in UTC, or in the local time
-  // zone
+  // isUtc and tiemzone together specify the timezone of this file;
+  // - if isUtc is true, timezone is ignored and this datetime is UTC.
+  // - if isUtc is false and timezone is not null, then this datetime
+  //   is in the given timezone
+  // - if isUtc is false and timezone is null, then this datetime
+  //   represents a floating time; the actual nativeTime representation
+  //   will still be as if it was in UTC
   attribute PRBool isUtc;
-
-  // if true, this calIDateTime represents a date (whole day), not a
-  // specific second
-  attribute PRBool isDate;
-
-  // an associated timezone that's the preferred way of representing
-  // this object.  If this is null/empty, then there is no preferred
-  // timezone, and UTC or localtime is assumed (based on isUtc).  If
-  // this is non-null when assigned, isUtc is automatically cleared.
   attribute AUTF8String timezone;
 
-  // the offset in minutes from UTC that this date is stored in.
-  attribute PRInt32 timezoneOffset;
+  // if true, this calIDateTime represents a date (whole day), not a
+  // specific hour/minute/second
+  attribute PRBool isDate;
 
   //
   // computed values
@@ -161,6 +160,12 @@ interface calIDateTime : nsISupports
   readonly attribute ACString icalString;
 
   /* JS only:
+   * // Setting jsDate via a JavaScript Date object will set
+   * // the calIDateTime to the jsDate's time ***in UTC***.
+   * // There's no way for us to recover TZ info from a jsDate,
+   * // so we always pull it out as UTC, and force the calIDateTime's
+   * // timezone to UTC.
    * attribute Date jsDate;
+   *
    */
 };
diff --git a/calendar/base/src/calDateTime.cpp b/calendar/base/src/calDateTime.cpp
index 138ff0fb4af0..f3357e6fd074 100644
--- a/calendar/base/src/calDateTime.cpp
+++ b/calendar/base/src/calDateTime.cpp
@@ -141,7 +141,6 @@ calDateTime::Reset()
     mIsUtc = PR_FALSE;
     mWeekday = 0;
     mYearday = 0;
-    mTimezoneOffset = 0;
     mIsDate = PR_FALSE;
 
     return NS_OK;
@@ -157,7 +156,6 @@ CAL_VALUETYPE_ATTR(calDateTime, PRInt16, Minute)
 CAL_VALUETYPE_ATTR(calDateTime, PRInt16, Second)
 CAL_VALUETYPE_ATTR(calDateTime, PRBool, IsUtc)
 CAL_VALUETYPE_ATTR(calDateTime, PRBool, IsDate)
-CAL_VALUETYPE_ATTR(calDateTime, PRInt32, TimezoneOffset)
 
 CAL_VALUETYPE_ATTR_GETTER(calDateTime, PRInt16, Weekday)
 CAL_VALUETYPE_ATTR_GETTER(calDateTime, PRInt16, Yearday)
@@ -296,7 +294,7 @@ calDateTime::GetInTimezone(const char *aTimezone, calIDateTime **aResult)
 
         ToIcalTime(&icalt);
 
-        if (!aTimezone) {
+        if (!aTimezone || strcmp(aTimezone, "UTC") == 0 || strcmp(aTimezone, "utc") == 0) {
             destzone = icaltimezone_get_utc_timezone();
         } else {
             nsCOMPtr<calIICSService> ics = do_GetService(kCalICSService);
@@ -596,21 +594,8 @@ calDateTime::SetProperty(nsIXPConnectWrappedNative *wrapper, JSContext * cx,
                 LL_MUL(utcTime, utcTime, thousands);
 
                 mIsUtc = PR_TRUE;
-
-                jsval tzoffsetval;
-                if (!JS_CallFunctionName(cx, dobj, "getTimezoneOffset", 0, nsnull, &tzoffsetval)) {
-                    mValid = PR_FALSE;
-                } else {
-                    JS_ValueToECMAInt32(cx, tzoffsetval, (int32*)&mTimezoneOffset);
-                    mTimezone.AssignLiteral("");
-                    nsresult rv = SetNativeTime(utcTime);
-
-                    if (NS_FAILED(rv)) {
-                        mValid = PR_FALSE;
-                    } else {
-                        mValid = PR_TRUE;
-                    }
-                }
+                mTimezone.Truncate();
+                mValid = PR_TRUE;
             }
 
             *_retval = PR_TRUE;
diff --git a/calendar/base/src/calDateTime.h b/calendar/base/src/calDateTime.h
index 222c849c930c..67840467fe10 100644
--- a/calendar/base/src/calDateTime.h
+++ b/calendar/base/src/calDateTime.h
@@ -47,6 +47,7 @@
 #include "calIDateTime.h"
 
 struct icaltimetype;
+struct _icaltimezone;
 
 class calDateTime : public calIDateTime,
                     public nsIXPCScriptable
@@ -81,12 +82,12 @@ protected:
     PRBool mIsUtc;
     PRBool mIsDate;
     nsCString mTimezone;
-    PRInt32 mTimezoneOffset;
 
     PRInt16 mWeekday;
     PRInt16 mYearday;
 
     void FromIcalTime(icaltimetype *icalt);
+    struct _icaltimezone* GetIcalTZ(const nsACString& tzid);
 
     PRTime mLastModified;
 };