§5.8611 — Even before timestamps were conceptually, and then practically, linked, a timestamp was already a ‘trusted timestamp’ if it was anything. Verifiable dating of digital documents poses a problem closely analogous to that of digital money, brought to a point of criticality by the ease of perfect replication. In both cases, initial solutions involved procedures of formal vouching by trusted third parties. For timestamps, the role of supervised banks is taken by Time Stamping Authorities (TSAs). Public Key Cryptography is employed to render time-stamps indelible – resistant to modification by anyone accessing the document in question, including its creator.
§5.86111 — Linked timestamping draws primarily on work by Haber and Stornetta, dating back to the beginning of the 1990s. This work was directed towards secure notarization, which is to say the verification – within a digital environment – of a document’s historical existence, with special reference to questions of priority. A facility of this kind has obvious relevance to legal documents, such as contracts and intellectual property claims. Linking timestamps adds dynamic to the procedure, by extending it to digital entities undergoing successive modification, such as changing inventories, and accounts. At each (discrete) stage of transformation, an additional timestamp is signed, or (in later versions) hashed, constituting a chain, pointing into an increasingly edit-resistant past. Each timestamp in the chain envelops the preceding series. It thus establishes public order, or absolute succession, in which the past is uncontroversial, and secure. As Satoshi Nakamoto notes in the Bitcoin paper, “Each timestamp includes the previous timestamp in its hash, forming a chain, with each additional timestamp reinforcing the ones before it.”
§5.86112 — A series of linked timestamps is already, at least in embryo (or larva), a ‘block-chain’. The stamps operate as irreducible moments, whose order is settled (immanently) by embedding. Their time is sheer order, without cardinality. Any timestamping system nevertheless inherits a time-keeping procedure, amounting to a fully-functional calendar, whose granulated ‘dates’ it competently codes. Unix time is the most widely applied system of this kind. Bitcoin adopts it.
§5.86113 — Taking
timestamping into trustlessness was a development that had to await Bitcoin.
While linked timestamping provides the basic architecture for secure
(edit-resistant) ledgers, their robust decentralization depends upon additional
cryptographic advances, supporting validation, compression, and consensus.
 As the Internet Society remarks in 2001, in proposing the RFC 3161 Internet X.509 Public Key Infrastructure Time-Stamp Protocol: “In order to associate a datum with a particular point in time, a Time Stamp Authority (TSA) may need to be used. This Trusted Third Party provides a ‘proof-of-existence’ for this particular datum at an instant in time.”
 See: Haber, S. and Stornetta, W.S. ‘How to time-stamp a digital document’ (1991)
 Unix time counts forwards, in seconds, from 00:00:00, January 1, 1970, (a Thursday). It ignores leap seconds, treating the length of each day as 86,400 seconds. It therefore gradually drifts from Universal Time.
When encoded in 32-bit format this time system reaches (Y2K-type) crisis on January 19, 2038. This poses no direct threat to Bitcoin, which employs a fully future-competent 64-bit Unix time code.
 See (for e.g.): Bela Gipp, Norman Meuschke, and André Gernandt, ‘Decentralized Trusted Timestamping using the Crypto Currency Bitcoin’ (National Institute of Informatics Tokyo, Japan, 2015)