blf: interpret object timestamps as signed 64-bit values

Change BLF object header timestamp fields from uint64 (Q)
to int64 (q) for both V1 and V2 object headers.

Observed CANape-generated BLF files containing messages
before the recording start timestamp. When interpreted as
unsigned values, these timestamps wrap around and appear
approximately 2^64 ns in the future, resulting in invalid
timestamps (e.g. year 2611) and NaT values in pandas.

Using signed 64-bit timestamps correctly preserves negative
recording-relative offsets while still providing a usable
range of approximately ±292 years at nanosecond resolution.

Root cause:
 
A timestamp of -60 s was decoded as
 
	2^64 ns - 60 s
 
which produced timestamps approximately 584 years in the
future. The resulting values exceeded the valid pandas
datetime range and appeared as NaT.

Author: FlorianMickler

can/io/blf.py

@@ -49,10 +49,10 @@ class BLFParseError(Exception):
 OBJ_HEADER_BASE_STRUCT = struct.Struct("<4sHHLL")
 
 # flags, client index, object version, timestamp
-OBJ_HEADER_V1_STRUCT = struct.Struct("<LHHQ")
+OBJ_HEADER_V1_STRUCT = struct.Struct("<LHHq")
 
 # flags, timestamp status, object version, timestamp, (original timestamp)
-OBJ_HEADER_V2_STRUCT = struct.Struct("<LBxHQ8x")
+OBJ_HEADER_V2_STRUCT = struct.Struct("<LBxHq8x")
 
 # compression method, size uncompressed
 LOG_CONTAINER_STRUCT = struct.Struct("<H6xL4x")

doc/changelog.d/+blf_prerecording_timestamps.fixed.rst

@@ -0,0 +1 @@
+changed interpretion of blf recording relative timestamps from unsigned to signed int64