retroactive

1. ADJ If a decision or action is retroactive, it is intended to take effect from a date in the past. (決定或行爲)有追溯傚力的[正式]

2. retroactivelyADV [ADV with v]

It isn't yet clear whether the new law can actually be applied retroactively. 這項新法令是否具有追溯力還不清楚。