Base, the Ethereum layer 2 network built by Coinbase, activated its Denim upgrade at 18:00 UTC on Thursday (1 p.m. ET), the time Base scheduled on its status page. Base’s first 200-millisecond block, No. 53,556,127, landed at 18:00:01 UTC, two seconds after the last two-second block, according to the network’s public RPC data checked by Unchained. That is ten blocks for every one Base made before: a new block each 200 milliseconds rather than each two seconds. Base said Denim was “officially live on mainnet” in a post on X about two hours later.

That speed-up turns any clock counted in blocks into a much shorter one. Started after activation, a timelock written as 302,400 blocks, a week at two-second blocks, would run out in about 16.8 hours, by Unchained’s math. Base’s Denim specification warns that a system “that treats a block count as a duration will see that duration shrink to one tenth of its intended length after activation.” Governance votes counted in blocks, vesting schedules, cooldowns and expiry deadlines are all on its audit list. Contracts that read `block.timestamp` keep their timing, but a block count fixed inside an immutable contract cannot be adjusted now that Denim is live.

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What Changes for Apps

From July 2025 until Thursday, Base ran Flashblocks, a system built with Flashbots that pushed out partial results every 200 milliseconds as each two-second block filled, which apps could treat as preconfirmations. Denim ended that. Now each of those 200-millisecond pieces is a block in its own right, numbered and taken through the usual unsafe, safe and finalized stages.

“Denim replaces Flashblocks, so applications using Flashblocks must migrate to canonical block and RPC streams,” Base’s Denim overview says. Base’s migration guide maps the old Flashblocks subscriptions to standard ones and says the `pending` block tag has “no direct equivalent after Denim.” Uniswap V3 style price oracles keep correct values, according to the spec, but a fixed number of stored observations may cover “as little as half as much wall-clock history.”

Contracts can read a block’s milliseconds through a new BaseTime contract, since `block.timestamp` still counts only whole seconds; RPC responses gain optional millisecond timestamp fields too.

Who Has to Act

Node operators must run base/base v1.5.1 or later; Base released v1.5.1 on Oct. 22 as the required version for Denim. A later release, v1.5.3, fixed a separate state root bug. Fourteen minutes after activation, Base said on its status page that Denim had “successfully activated” and that the team was investigating “periodic transaction inclusion delays, causing a higher than expected rate of empty blocks.” The page had posted no further update as of 7:30 p.m. ET. In Unchained’s check of Base’s public RPC data, 146 of the first 300 blocks after activation carried only system transactions; samples of 300 blocks taken hourly from 4 p.m. ET through 7 p.m. ET had none.

Base’s upgrades page lists three named upgrades before Denim this year: Azul in May, Beryl in June and Cobalt in September. Everest, the next entry, has no date yet.

One chain said Denim led it to speed up its shutdown. Horizen, a layer 3 that runs on the OP Stack and settles to Base, said in October that its chain “cannot follow” Denim and that, according to its infrastructure provider, no fix exists. In Horizen’s words, “about 40 minutes after activation the L3 can no longer include user transactions, permanently.” Users had until 13:00 UTC on Nov. 4 to move assets from the L3 to Base, and the Horizen DAO plans to send remaining ZEN and ETH balances to holders on Base.

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