ago; dating back from now
A formal, somewhat literary expression used to indicate how long ago something happened. Typically followed by a time period, e.g., 今を去ること10年前.
Twenty years ago, this town was a small village.
We pass down events that happened half a century ago.
A set phrase composed of 今 (now), を (object marker), 去る (to pass, to leave), and こと (nominalizer). Literally 'the fact of leaving now', it has become a fixed expression meaning 'ago'.