When a transaction takes place, such as a school issuing a record to a student, a small fee is paid by the issuer so the transaction gets logged in a blockchain. That happens at a certain point in time, so the timestamp is generated by the blockchain when that event occurs.
Blockcerts launched using Bitcoin in 2016. Then it expanded to included Ethereum. In 2018, we’ve seen it used on Hyperledger as well. It is architected to be adaptable, so feel free to get involved with the open source community to help it expand.