Time mines were temporal explosives that could be programmed to explode at some point in the future as well as in the past.
Time mines were stored in Vassarian's Battle TARDIS. Heleyna, a Time Lady revealed to be a spy of the Daleks, used a time mine set to explode in the past as a distraction to allow her past self to escape to the star chamber with the War Ollistra as her hostage. (AUDIO: Eye of Harmony)