Fix get_dtl to read full 4-byte nanosecond field and clean up fuzz test

get_dtl was reading only byte 8 as a raw integer for microseconds,
but the DTL format stores nanoseconds as a 4-byte big-endian uint32
in bytes 8-11. This fix reads the full field and converts ns to us.
Also removes unnecessary KeyError catch from PDU fuzz test.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: gijzelaerr

snap7/util/getters.py

@@ -656,15 +656,38 @@ def get_ldt(bytearray_: bytearray, byte_index: int) -> datetime:
 
 
 def get_dtl(bytearray_: bytearray, byte_index: int) -> datetime:
+    """Get DTL (Date and Time Long) value from bytearray.
+
+    Notes:
+        Datatype ``DTL`` consists of 12 bytes in the PLC:
+        - Bytes 0-1: Year (uint16, big-endian)
+        - Byte 2: Month (1-12)
+        - Byte 3: Day (1-31)
+        - Byte 4: Weekday (1=Sunday, 7=Saturday)
+        - Byte 5: Hour (0-23)
+        - Byte 6: Minute (0-59)
+        - Byte 7: Second (0-59)
+        - Bytes 8-11: Nanoseconds (uint32, big-endian)
+
+    Args:
+        bytearray_: buffer to read from.
+        byte_index: byte index from where to start reading.
+
+    Returns:
+        datetime value (microsecond precision; sub-microsecond nanoseconds are truncated).
+    """
+    nanoseconds = struct.unpack(">I", bytearray_[byte_index + 8 : byte_index + 12])[0]
+    microsecond = nanoseconds // 1000
+
     time_to_datetime = datetime(
         year=int.from_bytes(bytearray_[byte_index : byte_index + 2], byteorder="big"),
         month=int(bytearray_[byte_index + 2]),
         day=int(bytearray_[byte_index + 3]),
         hour=int(bytearray_[byte_index + 5]),
         minute=int(bytearray_[byte_index + 6]),
         second=int(bytearray_[byte_index + 7]),
-        microsecond=int(bytearray_[byte_index + 8]),
-    )  # --- ? noch nicht genau genug
+        microsecond=microsecond,
+    )
     if time_to_datetime > datetime(2554, 12, 31, 23, 59, 59):
         raise ValueError("date_val is higher than specification allows.")
     return time_to_datetime

tests/test_hypothesis.py

@@ -302,24 +302,12 @@ def test_dt_roundtrip(value: datetime) -> None:
     )
 )
 def test_dtl_roundtrip(value: datetime) -> None:
-    # DTL stores nanoseconds derived from microseconds, but get_dtl reads
-    # byte 8 as raw microsecond value. Truncate to match.
-    # set_dtl writes microsecond * 1000 as nanoseconds (4 bytes big-endian)
-    # get_dtl reads byte 8 as int — this is the first byte of the 4-byte nanosecond field
-    nanoseconds = value.microsecond * 1000
-    ns_bytes = struct.pack(">I", nanoseconds)
-    expected_microsecond = ns_bytes[0]  # get_dtl reads only byte_index+8
-
+    # DTL stores nanoseconds (microsecond * 1000), so the roundtrip
+    # preserves microsecond precision exactly.
     data = bytearray(12)
     set_dtl(data, 0, value)
     result = get_dtl(data, 0)
-    assert result.year == value.year
-    assert result.month == value.month
-    assert result.day == value.day
-    assert result.hour == value.hour
-    assert result.minute == value.minute
-    assert result.second == value.second
-    assert result.microsecond == expected_microsecond
+    assert result == value
 
 
 # ---------------------------------------------------------------------------
@@ -530,7 +518,7 @@ def test_pdu_parse_no_crash(data: bytes) -> None:
     proto = S7Protocol()
     try:
         proto.parse_response(data)
-    except (S7ProtocolError, struct.error, ValueError, IndexError, KeyError):
+    except (S7ProtocolError, struct.error, ValueError, IndexError):
         pass  # Expected for malformed data