Patent ID: 7139226

Claim:
A time indicator comprising: a front part comprising an opaque layer and a colorant layer in contact with the opaque layer at an interface, the colorant layer comprising a matrix and a colorant in the matrix, the colorant having a non-migratory form in which the colorant does not migrate in the matrix to the interface and a migratory form in which the colorant migrates in the matrix to the interface; and a back part comprising a reactant capable of migrating in the colorant layer, wherein, when the front part and the back part are placed in contact, the reactant migrates into the colorant layer and reacts with the non-migratory form of the colorant converting the non-migratory form of the colorant to the migratory form of the colorant such that the migratory form of the colorant migrates to the interface and through the opaque layer to cause a visual color indication in the front part.