Precise time, frequency and phase are all key elements in critical sectors such as Power and Utility Companies, Financial and Banking markets, Mobile and Computer Networks . More and more devices and systems as well as critical infrastructure and networks rely on GPS or on other GNSS for providing accurate timing and synchronization. GPS signals are precariously weak and can easily be blocked, jammed or spoofed, intentionally or unintentionally. Such attacks are recognized as posing a serious threat to many critical infrastructure applications which currently rely heavily on the publicly available GPS signal. Time FireWall is a box that is inserted between the antenna and the existing GNSS receiver. The TFW™ receives the GNSS satellite signal from an antenna, checks the integrity of the signal using an internal atomic clock and various techniques and when it determines that the GNSS signal is reliable it passes an RF signal on to the timing network with “a seal of integrity”. If the Time FireWall determines that the GNSS signal is unreliable (either due to blocking or jamming or spoofing or any other malicious attacking), the TFW sends out a warning alarm and uses its internal satellite signal simulator and Rubidium Atomic Clock to provide an alternate GNSS signal to the customers receiver, allowing continuous and uninterrupted operation of timing and synchronization even in a GNSS denied or spoofed environment. The TFW™ can be easily inserted into any timing network and is independent of the manufacturer of the timing solution already installed at the customer’s site. No changes are required in the legacy equipment already installed except for some very minimal changes in cabling.