Truncate milliseconds to i64 before serializing. #1755

Job	Run time
Type-check benchmarks	32s
Type checking (MSRV, no_std)	20s
Test (miri)	20m 24s
Type checking (MSRV, std_no_offset)	7s
Test (Ubuntu, MSRV)	48s
Test (Windows, MSRV)	32s
Type checking (MSRV, std_with_offset)	6s
Clippy	4s
Test (MacOS, MSRV)	51s
Type checking (stable, no_std)	6s
Test (Ubuntu, stable)	36s
Type checking (stable, std_no_offset)	5s
Test (Windows, stable)	32s
Cross-build	1m 39s
Type checking (stable, std_with_offset)	17s
Test (MacOS, stable)	32s
Documentation	19s
Formatting	11s
Coverage	54s
Publish docs	0s
	28m 55s

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