According to the UEFI spec, the timezone setting which the platform needs
to record in addition to the actual date and time already reflects the
current DST setting. In other words, moving the clock from standard time
to daylight saving time also involves adding or subtracting 60 minutes
from the timezone setting, as well as flicking the EFI_TIME_IN_DAYLIGHT
bit in the DST setting.

This means we need to disregard the DST setting if the timezone is
specified, and only add or subtract the additional hour if we are on
local time.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
---
 ArmPlatformPkg/Library/PL031RealTimeClockLib/PL031RealTimeClockLib.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/ArmPlatformPkg/Library/PL031RealTimeClockLib/PL031RealTimeClockLib.c b/ArmPlatformPkg/Library/PL031RealTimeClockLib/PL031RealTimeClockLib.c
index f1eb0deb3249..459dcc0a0519 100644
--- a/ArmPlatformPkg/Library/PL031RealTimeClockLib/PL031RealTimeClockLib.c
+++ b/ArmPlatformPkg/Library/PL031RealTimeClockLib/PL031RealTimeClockLib.c
@@ -164,12 +164,10 @@ LibGetTime (
   }
 
   // Adjust for the correct time zone
+  // The timezone setting also reflects the DST setting of the clock
   if (Time->TimeZone != EFI_UNSPECIFIED_TIMEZONE) {
     EpochSeconds += Time->TimeZone * SEC_PER_MIN;
-  }
-
-  // Adjust for the correct period
-  if ((Time->Daylight & EFI_TIME_IN_DAYLIGHT) == EFI_TIME_IN_DAYLIGHT) {
+  } else if ((Time->Daylight & EFI_TIME_IN_DAYLIGHT) == EFI_TIME_IN_DAYLIGHT) {
     // Convert to adjusted time, i.e. spring forwards one hour
     EpochSeconds += SEC_PER_HOUR;
   }
@@ -229,12 +227,10 @@ LibSetTime (
   EpochSeconds = EfiTimeToEpoch (Time);
 
   // Adjust for the correct time zone, i.e. convert to UTC time zone
+  // The timezone setting also reflects the DST setting of the clock
   if (Time->TimeZone != EFI_UNSPECIFIED_TIMEZONE) {
     EpochSeconds -= Time->TimeZone * SEC_PER_MIN;
-  }
-
-  // Adjust for the correct period
-  if ((Time->Daylight & EFI_TIME_IN_DAYLIGHT) == EFI_TIME_IN_DAYLIGHT) {
+  } else if ((Time->Daylight & EFI_TIME_IN_DAYLIGHT) == EFI_TIME_IN_DAYLIGHT) {
     // Convert to un-adjusted time, i.e. fall back one hour
     EpochSeconds -= SEC_PER_HOUR;
   }
-- 
2.11.0

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