A timestamp proves that a message existed before a specific time. What you are describing actually does the opposite: proves that a message was created after a specific time. To do that, you can use random beacons: numbers that we know didn't exist prior to a specific point in time. Bitcoin block hashes make for good random beacons.

The problem is it's very hard to come up with schemes where random beacons are actually secure and useful; timestamps are much easier to actually use.

For timestamping, my OpenTimestamps tool is by far the easiest thing to use: https://OpenTimestamps.org