windows: fix filetime conversions and deprecate Nanoseconds

windows/registry/key.go

@@ -198,7 +198,20 @@ type KeyInfo struct {
 
 // ModTime returns the key's last write time.
 func (ki *KeyInfo) ModTime() time.Time {
-	return time.Unix(0, ki.lastWriteTime.Nanoseconds())
+	lastHigh, lastLow := ki.lastWriteTime.HighDateTime, ki.lastWriteTime.LowDateTime
+	// 100-nanosecond intervals since January 1, 1601
+	hsec := uint64(lastHigh)<<32 + uint64(lastLow)
+	// Convert _before_ gauging; the nanosecond difference between Epoch (00:00:00
+	// UTC, January 1, 1970) and Filetime's zero offset (January 1, 1601) is out
+	// of bounds for int64: -11644473600*1e7*1e2 < math.MinInt64
+	sec := int64(hsec/1e7) - 11644473600
+	nsec := int64(hsec%1e7) * 100
+	return time.Unix(sec, nsec)
+}
+
+// ModTimeZero reports whether the key's last write time is zero.
+func (ki *KeyInfo) ModTimeZero() bool {
+	return ki.lastWriteTime.LowDateTime == 0 && ki.lastWriteTime.HighDateTime == 0
 }
 
 // Stat retrieves information about the open key k.

windows/registry/registry_test.go

@@ -9,6 +9,7 @@ package registry_test
 import (
 	"bytes"
 	"crypto/rand"
+	"errors"
 	"os"
 	"syscall"
 	"testing"
@@ -674,3 +675,30 @@ func GetDynamicTimeZoneInformation(dtzi *DynamicTimezoneinformation) (rc uint32,
 	}
 	return
 }
+
+func TestModTimeZeroValue(t *testing.T) {
+	k, err := registry.OpenKey(registry.LOCAL_MACHINE, `SOFTWARE\Microsoft\Windows NT\CurrentVersion\Perflib\009`, registry.READ)
+	if err != nil {
+		if errors.Is(err, syscall.ERROR_FILE_NOT_FOUND) {
+			t.Skip("Perflib key not found; skipping")
+		}
+		t.Fatal(err)
+	}
+	defer k.Close()
+
+	// Modification time of Perflib keys is known to be set to
+	// Filetime's zero value: get stats and check.
+	stats, err := k.Stat()
+	if err != nil {
+		t.Fatal(err)
+	}
+	// First verify input is zero (assume ModTimeZero uses it directly).
+	if !stats.ModTimeZero() {
+		t.Error("Modification time of Perflib key should be zero")
+	}
+	// Then check ModTime directly thus conversion implicitly.
+	modTime := stats.ModTime()
+	if !modTime.Equal(time.Date(1601, time.January, 1, 0, 0, 0, 0, time.UTC)) {
+		t.Errorf("ModTime should be 1601-01-01, but is %v", modTime)
+	}
+}