What does release at block# means?

“Constantinople is now scheduled for mainnet release at block 7,080,000”… Not clear what release means here? Is main net like a server that will be updated with new code or does this simply means that all node will update their clients with new code?

Also, if it implies the latter, then why not propose a date and time rather than a block#?

Submitted December 21, 2018 at 02:43AM }
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