fix: add unix_epoch_second() with floor semantics for negative timestamps #986

Job	Run time
Minimum Supported Rust Version (windows-latest)	41s
Run the fuzzers	3m 3s
cargo check (windows-latest)	1m 42s
Code Coverage (macos-latest, macos)	1m 49s
cargo clippy -- -D warnings (windows-latest)	1m 31s
cargo clippy -- -D warnings (ubuntu-latest)	29s
Code Coverage (ubuntu-latest, unix)	1m 47s
Minimum Supported Rust Version (ubuntu-latest)	16s
cargo check (macOS-latest)	12s
Code Coverage (windows-latest, windows)	4m 58s
cargo fmt --all -- --check	5s
cargo clippy -- -D warnings (macOS-latest)	34s
Minimum Supported Rust Version (macOS-latest)	16s
cargo check (ubuntu-latest)	6s
cargo test (macOS-latest)	27s
cargo test (ubuntu-latest)	29s
cargo test (windows-latest)	1m 16s
	19m 41s

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