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Many pervasive and distributed systems require clocks synchronised to a unique and external reference time. Unfortunately, the actual difference between the time of a local clock and the reference time, called offset, is a variable factor that is very hard to compute: synchronisation mechanisms typically estimate the offset, without offering guarantees of closeness of the estimate to the true value. The paper gives a comprehensive description of the Reliable and Self-Aware Clock (R&SAClock), a new software clock that provides both the current time and the synchronisation uncertainty, that is a self-adaptive and conservative estimate of the offset, thus being capable of monitoring synchronisation quality and detecting clock failures. Two different solutions are presented for the evaluation of the synchronisation uncertainty which differ in terms of performance and coverage, and are evaluated in the context of distributed wearable mobile devices intended for railway trackside workers.