When Monero was launched, it used a 1 minute block time. In the question Why was the block time changed from 1 to 2 minutes?, reasons for the switch were discussed. When, however, did this switch actually occur?
I am wondering both the block height and date.
The change occured at hard fork 2, on block 1009827 (see the table near the top of src/cryptonote_core/blockchain.cpp). This was in march 2016.
The change was introduced with the Monero v0.9.0 "Hydrogen Helix" release on the 1st of Jan, 2016 however it did not take effect until the 20th of March, 2016 at block 1009827 as explained by user3603. More details about the original Hydrogen Helix release can be found here
