The concept of traceability for measurements of time of day refers to a continuous chain of comparisons extending from a time comparison, time stamp, or clock synchronization performed by a user, back through the distribution of one of the UTC(K) time scales, and so on to the reference time scale UTC. Each of these comparisons in the chain is carried out with some inherent level of inaccuracy (uncertainty of the measurement). The uncertainties of each link in the overall chain can then be combined to give the total uncertainty of the time signal at the point where it is available to the user. Understanding the dissemination chain and calculating this uncertainty is essential to meeting requirements of being able to demonstrate traceability. Each link in the chain must be documented, along with its associated uncertainty evaluation, and the resulting total uncertainty of the timestamp or time output must be determined and recorded. 

There are two additional requirements for demonstrating time traceability to a common reference. One is for the timing equipment to be calibrated so that its unknown internal delays do not bias its time output. The second is for the equipment to be monitored continuously so that any fault or anomaly can be detected, and the time output not used until the equipment is once again working correctly. The calibration evidence and monitoring results should be archived so that the status of the timing equipment at any given point in time can be verified later. 

## Overview of Regulations: Past, Present and Future 

Prior to the advent of High Frequency Trading (HFT) and distributed electronic trading platforms, many of the clocks maintained for financial transactions were mechanical devices that physically stamped the time (ink) onto paper documents used to record the transactions. These clocks were seldom synchronized to the granularity expected today. By the late 1990s it became obvious that the performance of these clocks was far too limited for financial markets, and synchronization requirements were implemented in the U.S. and in Europe. Since there are some substantial differences, let’s examine the U.S. and EU requirements in separate sub-sections below. 

## United States: NASD, OATS, FINRA, CAT 

In August 1996, the U.S. Securities and Exchange Commission (SEC) issued a report alleging that the National Association of Securities Dealers (NASD) and NASDAQ did not always act in the best interest of customers. This report led to a financial settlement between the SEC and NASD that also included new regulations. To comply with the new regulations, the NASD issued a series of rules that were approved by the SEC in March 1998 and went into effect in August 1998. One rule was NASD OATS (Order Audit Trail System) Rule 6953, entitled "Synchronization of Member Business Clocks". Rule 6953 required computer systems and mechanical clocks to be synchronized every business day before the stock market opened to ensure that event time stamps are accurate. It also required the synchronization to be within three seconds of the NIST atomic clock, which while not particularly stringent, did finally force the removal of old clocks that could not display seconds. The NYSE adopted similar requirements in 2003. 

The US based Financial Industry Regulatory Authority (FINRA) is a not for profit entity that is responsible for overseeing US brokerage firms and works closely with the SEC. In 2008, FINRA issued new requirements for stock market time synchronization. These new requirements, contained in FINRA OATS Rule 7430 "Synchronization of Member Business Clocks", superseded Rule 6953 for the NASDAQ (2008) and for the NYSE (2011). Similar to the previous requirements of Rule 6953, Rule 7430 lists NIST time as the official reference for U.S. stock market transactions. Rule 7340 made the synchronization requirement more 

