Let's say I wanted to know what the exact blockheight of the Monero blockchain was on March 21st 2019. Is there a way to do this or an easy to follow chart or something?
Monero targets a new block every 2 minutes, so if we set H as the current block height, N as the current timestamp, T as the past timestamp of the datetime you want to know the block height of, then:
H - (N - T) / 120
Gets you very near. Blocks aren't mined exactly every 2 minutes, so there will be fluctuation. To narrow down further, you can then use a block explorer like https://xmrchain.net/, or your local node.
For example, trying to find the first block mined on March 21st 2019:
1885896 - (1564019260 - 1553126400) / 120 = 1795122