You are looking at a Commitment Transaction. Commitment transactions are submitted to the network by one channel owner to unilaterally close a Lightning Network channel. The locktime in commitment transactions is used to communicate channel state which results in all commitment transactions being locked to a date in 1987.
Bolt3 specifies that commitment transactions store an obscured commitment transaction number (i.e. the “edition” of the commitment) split across the locktime and sequence fields. This makes it easier for the channel owner to identify whether the latest or an older commitment transaction was broadcast by the counterparty.
The commitment transaction number is not stored in plaintext but obscured by XORing it with a shared secret created by hashing the concatenated payment_basepoint from the open_channel and accept_channel messages. Since the channel owners know these basepoints, they can revert the XOR to read the encoded data, but the data appears pseudorandom to any third parties.
In both the locktime and sequence, the upper byte is fixed, while the lower three bytes each store three bytes of the obscured commitment transaction number. In the locktime the upper byte must be set to 0x20, so the locktime of a commitment transaction must always fall in [0x20000000, 0x20ffffff], i.e.
53687091210 ≤
locktime≤ 55364812710
which will always results in a time-based locktime to a date between 1987-01-05 and 1987-07-18.
