Constant MAX_EXPIRY_TIMESTAMP_MS 

pub const MAX_EXPIRY_TIMESTAMP_MS: u128 = 32_503_680_000_000; // 32_503_680_000_000u128

Latest accepted expiry, 3000-01-01T00:00:00Z in epoch milliseconds.

An expiry is only ever bounded below, against now. Downstream the receipt renders it as rfc3339, which has four year digits, so an unbounded value has no representable form and fails serialization while answering the client — after the transaction is already WAL-durable. Year 3000 is far past any real order and far inside the range the renderer covers.