Constant bc::LOCKTIME_THRESHOLD

pub const LOCKTIME_THRESHOLD: u32 = 500_000_000;

The Threshold for deciding whether a lock time value is a height or a time (see Bitcoin Core).

LockTime values below the threshold are interpreted as block heights, values above (or equal to) the threshold are interpreted as block times (UNIX timestamp, seconds since epoch).

Bitcoin is able to safely use this value because a block height greater than 500,000,000 would never occur because it would represent a height in approximately 9500 years. Conversely, block times under 500,000,000 will never happen because they would represent times before 1986 which are, for obvious reasons, not useful within the Bitcoin network.