Fix for DST ValidStarting time not checked (#28188)

Previous code didn't check the starting time for the DST and neither did the test. This adds the check to the code and the test, will need to add to the test plans as well.

src/app/clusters/time-synchronization-server/time-synchronization-server.cpp

@@ -571,7 +571,10 @@ CHIP_ERROR TimeSynchronizationServer::GetLocalTime(EndpointId ep, DataModel::Nul
     timeZoneOffset       = static_cast<int64_t>(tzStore.timeZone.offset);
     VerifyOrReturnError(GetDSTOffset().size() != 0, CHIP_ERROR_INVALID_TIME);
     const auto & dst = GetDSTOffset()[0];
-    dstOffset        = static_cast<int64_t>(dst.offset);
+    if (dst.validStarting <= chipEpochTime)
+    {
+        dstOffset = static_cast<int64_t>(dst.offset);
+    }
 
     uint64_t usRemainder = chipEpochTime % chip::kMicrosecondsPerSecond;   // microseconds part of chipEpochTime
     chipEpochTime        = (chipEpochTime / chip::kMicrosecondsPerSecond); // make it safe to cast to int64 by converting to seconds

src/python_testing/TC_TIMESYNC_2_8.py

@@ -17,7 +17,7 @@
 
 import time
 import typing
-from datetime import timedelta
+from datetime import datetime, timedelta, timezone
 
 import chip.clusters as Clusters
 from chip.clusters.Types import NullValue
@@ -174,7 +174,23 @@ async def test_TC_TIMESYNC_2_8(self):
         local = await self.read_ts_attribute_expect_success(local_attr)
         compare_time(received=local, offset=timedelta(seconds=-3600), tolerance=timedelta(seconds=5))
 
-        self.print_step(27, "Send SetDSTOffset command")
+        self.print_step(27, "Send SetDSTOffset command with DST starting in the future")
+        valid = utc_time_in_matter_epoch(datetime.now(tz=timezone.utc) + timedelta(seconds=10))
+        dst = [dst_struct(offset=3600, validStarting=valid, validUntil=NullValue)]
+        await self.send_set_dst_cmd(dst)
+
+        self.print_step(28, "Read Localtime")
+        local = await self.read_ts_attribute_expect_success(local_attr)
+        compare_time(received=local, offset=timedelta(seconds=0), tolerance=timedelta(seconds=5))
+
+        self.print_step(29, "Wait 15s")
+        time.sleep(15)
+
+        self.print_step(30, "Read Localtime")
+        local = await self.read_ts_attribute_expect_success(local_attr)
+        compare_time(received=local, offset=timedelta(seconds=3600), tolerance=timedelta(seconds=5))
+
+        self.print_step(31, "Send SetDSTOffset command")
         dst = [dst_struct(offset=0, validStarting=0, validUntil=NullValue)]
         await self.send_set_dst_cmd(dst)