Standard arguments against backward time travel typically rely on thermodynamic improbability orcausality paradoxes. In this note, we present a more fundamental barrier based on the operationaldefinition of internal time (clockability). We argue that, within systems capable of maintaining noncancelable internal registers, backward time travel is not merely dynamically forbidden, butoperationally ill-posed.Because clockability is defined by the irreversible accumulation of state differences, any operationintended to reverse the temporal coordinate would require the coherent erasure of the very registersthat define the system’s history. Such an operation forces the system outside its domain of validity,rendering the comparison between the “returned” system and any past state physically meaningless.This framework explains why time-reversal phenomena (e.g. spin echoes or recurrences) arerestricted to regimes lacking full clockability and establishes an informational no-go result formacroscopic temporal inversion.