diff --git a/airframe-log/.native/src/main/scala/wvlet/log/LogTimestampFormatter.scala b/airframe-log/.native/src/main/scala/wvlet/log/LogTimestampFormatter.scala
index 7b7904a920..0951abe200 100644
--- a/airframe-log/.native/src/main/scala/wvlet/log/LogTimestampFormatter.scala
+++ b/airframe-log/.native/src/main/scala/wvlet/log/LogTimestampFormatter.scala
@@ -30,7 +30,7 @@ object LogTimestampFormatter {
       !ttPtr = (timeMillis / 1000).toSize
       val tmPtr = alloc[tm]()
       localtime_r(ttPtr, tmPtr)
-      val bufSize        = 26.toUSize
+      val bufSize        = 29.toUSize // max size for time strings
       val buf: Ptr[Byte] = alloc[Byte](bufSize)
       strftime(buf, bufSize, pattern, tmPtr)
       val ms = timeMillis % 1000
@@ -41,7 +41,12 @@ object LogTimestampFormatter {
 
       val tzBuf: Ptr[Byte] = alloc[Byte](5)
       strftime(tzBuf, 5.toUSize, c"%z", tmPtr)
-      strcat(buf, tzBuf)
+      if (strlen(tzBuf) <= 1.toUSize) {
+        // For UTC-00:00
+        strcat(buf, c"Z")
+      } else {
+        strcat(buf, tzBuf)
+      }
       fromCString(buf)
     }
   }