Missing Time Syndrome was an old Earth name given to a condition where individuals were not capable of remembering parts of their life.
This was typically done by advanced races when studying certain individuals and blanked their memories so they would not remember the encounter. The Reticulii, the First Contact Office and forms of mind control research were capable of accomplishing this affect.
In late 2374 when Quark could not remember the Cardassian Day of Withdrawal, it was suggested that perhaps he was suffering from Missing Time Syndrome due to Cardassian experiments. (DS9 - Millennium novel: The Fall of Terok Nor)