Is Bitcoin a decentralized time-stamping authority? Can Bitcoin/blockchain be thought of as a decentralized time-stamping authority (TSA)? Is there true consensus in the Bitcoin network on a mined block's time, or just that the block was mined?
Yes, Bitcoin can act as a time-stamping authority. However, depending on your application the accuracy of the Bitcoin blocks may not be sufficient.

Nodes only reject blocks that are greater than 2 hours into the future, and blocks created earlier than the median time of the past 11 blocks. Therefore, timestamps may be off by several hours.

also see: How accurate is Bitcoin network time?