You can publish the hash in some durable medium, like the classified section of a newspaper.
This proves you generated it before this time.
You can also include in the hash the close of the stock market and all the sports scores from the previous day. That proves you generated it after that time.
If you are looking to prove that something happened after a certain timestamp, you can use a randomness beacon[0]. Every <interval>, the beacon outputs a long random number. Include the timestamped random number into your artifact.
You are relying upon the authority of the beacon to be random, but good practice is to utilize multiple independent beacons.
This proves you generated it before this time.
You can also include in the hash the close of the stock market and all the sports scores from the previous day. That proves you generated it after that time.