Reapply "Refactor Date implementation in VM."

runtime/vm/os.h

Index: runtime/vm/os.h
diff --git a/runtime/vm/os.h b/runtime/vm/os.h
index 490098384157069746296281526e4ef25edb784c..43c680d60750e0da1fc7ccb6a18833078ddfed24 100644
--- a/runtime/vm/os.h
+++ b/runtime/vm/os.h
@@ -18,40 +18,19 @@ class Isolate;
 // Interface to the underlying OS platform.
 class OS {
  public:
-  // Takes the seconds since epoch (midnight, January 1, 1970 UTC) and breaks it
-  // down into date and time in the UTC timezone.
-  // The returned year is offset by 1900. The returned month is 0-based.
-  // Returns true if the conversion succeeds, false otherwise.
-  static bool GmTime(int64_t seconds_since_epoch, tm* tm_result);
-
-  // Takes the seconds since epoch (midnight, January 1, 1970 UTC) and breaks it
-  // down into date and time in the local time.
-  // The returned year is offset by 1900. The returned month is 0-based.
-  // Returns true if the conversion succeeds, false otherwise.
-  static bool LocalTime(int64_t seconds_since_epoch, tm* tm_result);
-
-  // Takes the broken down date and time in UTC timezone and computes the
-  // seconds since epoch (midnight, January 1, 1970 UTC).
-  // The given year is offset by 1900. The given month is 0-based.
-  // Returns true if the conversion succeeds, false otherwise.
-  static bool MkGmTime(tm* tm, int64_t* seconds_result);
-
-  // Takes the broken down date and time in local timezone and computes the
-  // seconds since epoch (midnight, January 1, 1970 UTC).
-  // The given year is offset by 1900. The given month is 0-based.
-  // Returns true if the conversion succeeds, false otherwise.
-  static bool MkTime(tm* tm, int64_t* seconds_result);
-
   // Returns the abbreviated time-zone name for the given instant.
   // For example "CET" or "CEST".
-  static bool GetTimeZoneName(int64_t seconds_since_epoch,
-                              const char** name_result);
+  static const char* GetTimeZoneName(int64_t seconds_since_epoch);
 
   // Returns the difference in seconds between local time and UTC for the given
   // instant.
   // For example 3600 for CET, and 7200 for CEST.
-  static bool GetTimeZoneOffsetInSeconds(int64_t seconds_since_epoch,
-                                         int* offset_result);
+  static int GetTimeZoneOffsetInSeconds(int64_t seconds_since_epoch);
+
+  // Returns the difference in seconds between local time and UTC when no
+  // daylight saving is active.
+  // For example 3600 in CET and CEST.
+  static int GetLocalTimeZoneAdjustmentInSeconds();
 
   // Returns the current time in milliseconds measured
   // from midnight January 1, 1970 UTC.