What is expiry time represents in this format

[Santhosh Kumar Gunturu]

I see the output.

root@X10SDV-8C-TLN4F:/mnt/config# cat
/var/lib/libvirt/dnsmasq/mgmt-1br1.status
[
  {
    "ip-address": "192.168.27.8",
    "mac-address": "52:54:00:42:21:14",
    "hostname": "vyatta",
    "expiry-time": 1589500228
  }
]

Can you please explain what does the expiry-time mean ? What are its units ?
Please let me know.

Thanks & Regards
Santhosh Kumar Gunturu