Decryption possible only within a period of time
This is possible, but only if you introduce one or more parties in addition to the sender and receiver. Also, the receiver must be online when decrypting.

Typically, the security requirements would be that (a) if the recipient tries to decrypt inside the time window, he should be able to decrypt, even if the sender is not cooperating; (b) if the recipient did not decrypt the message inside the time window, he should not be able to learn anything about the contents of the ciphertext; and (c) any additional parties should not learn anything about the ciphertext.

From such requirements, it should be fairly straight-forward to come up with reasonable schemes. Some variant of threshold decryption should do the trick.