docs: feat(base-time): expose the BaseTime predeploy read surface (base-std@740ee5d)

docs/base-chain/specs/reference/b20/changelog/03-denim-basetime-millisecond-timestamp.mdx

@@ -0,0 +1,69 @@
+---
+title: "BaseTime Millisecond Timestamp"
+description: "Denim adds the BaseTime predeploy so contracts can read the block timestamp at millisecond precision through a read-only interface."
+---
+
+## Abstract
+
+Denim installs the BaseTime predeploy at `0x4200000000000000000000000000000000000030`. It exposes the millisecond component of the current block timestamp. Base Std adds the read-only `IBaseTime` interface and a typed handle, `StdPredeploys.BASE_TIME`. Call `StdPredeploys.BASE_TIME.timestampMs()` for the full millisecond timestamp.
+
+## Motivation
+
+`block.timestamp` has second precision, but from Denim a block's timestamp carries a sub-second component. Contracts that need the exact block time, such as auctions, rate limits, or time-weighted pricing, can't read the millisecond component from the EVM alone.
+
+## What changed
+
+BaseTime is a proxied predeploy: Solidity bytecode at a `0x4200…` address, not a native precompile. Its handle therefore lives in a new `StdPredeploys` library instead of `StdPrecompiles`.
+
+### New interface and library
+
+```solidity title="IBaseTime and StdPredeploys" lines expandable
+interface IBaseTime {
+    function timestampMillisPart() external view returns (uint16);
+    function timestampMs() external view returns (uint64);
+    function version() external view returns (string memory);
+}
+
+library StdPredeploys {
+    address internal constant BASE_TIME_ADDRESS = 0x4200000000000000000000000000000000000030;
+    IBaseTime internal constant BASE_TIME = IBaseTime(BASE_TIME_ADDRESS);
+}
+```
+
+| Function | Selector | Status | Notes |
+| -------- | -------- | ------ | ----- |
+| `timestampMillisPart()` | `0x7b2fea99` | New (view) | Millisecond component: `0`, `200`, `400`, `600`, or `800`. Never reverts. |
+| `timestampMs()` | `0x5745a677` | New (view) | `block.timestamp * 1000 + timestampMillisPart()`. Never reverts. |
+| `version()` | `0x54fd4d50` | New (view) | Returns `"1.0.0"`. |
+
+The depositor-only setter and its errors are protocol-internal and are not part of `IBaseTime`.
+
+### Behavior
+
+- No existing selector, event, error, or storage slot changes. The predeploy is new.
+- Storage: `uint16 timestampMillisPart` sits in the low-order 2 bytes of slot 0.
+- The protocol writes the millisecond component once per Denim block, through a depositor-only setter called by the block-scoped deposit at `tx[1]`. Until that deposit executes (for example, during the `tx[0]` L1-info deposit), `timestampMs()` combines the current block's seconds with the previous block's millisecond component.
+
+### Example
+
+```solidity title="Read the millisecond timestamp" highlight={3}
+import {StdPredeploys} from "base-std/StdPredeploys.sol";
+
+uint64 nowMs = StdPredeploys.BASE_TIME.timestampMs();
+// block.timestamp == 1_800_000_000, timestampMillisPart() == 400
+// nowMs == 1_800_000_000_400
+```
+
+## Migration
+
+This change is additive. No existing integration changes.
+
+- After Denim activates, read `StdPredeploys.BASE_TIME.timestampMs()` for millisecond block time.
+- Code that runs before `tx[1]` in a block, such as the L1-info deposit, sees the previous block's millisecond component.
+- Before Denim, the predeploy has no implementation and calls to it revert. Don't depend on it on a chain where Denim hasn't activated.
+
+## Alternatives considered
+
+- **Read-only interface.** Only the protocol deposit can write the value, so exposing the setter would give integrators a selector that always reverts for them.
+- **Separate `StdPredeploys` library.** Predeploys are EVM bytecode and precompiles are native code. Separate libraries keep that distinction visible at the call site.
+- **Slot-0 mock layout.** `MockBaseTime` stores the `uint16` at slot 0 instead of an ERC-7201 namespace. This matches the real contract's layout, so `vm.store`-based tests behave the same against either.

docs/docs.json

@@ -470,7 +470,8 @@
               "upgrades/denim/migrate-from-flashblocks",
               "base-chain/specs/reference/b20/changelog/03-denim-b20-token-receiver",
               "base-chain/specs/reference/b20/changelog/03-denim-b20-transfer-executor-enforcement",
-              "base-chain/specs/reference/b20/changelog/03-denim-policyregistry-not-policy"
+              "base-chain/specs/reference/b20/changelog/03-denim-policyregistry-not-policy",
+              "base-chain/specs/reference/b20/changelog/03-denim-basetime-millisecond-timestamp"
             ]
           },
           {

docs/specifications/b20/changelog.mdx

@@ -24,6 +24,7 @@ B20 method and event signatures are part of the chain's consensus surface. Exist
 | B20 Asset, B20 Stablecoin | (Breaking) Reject the token itself as a credit recipient | `IB20` (shared surface), `IB20Asset` (`batchMint`) | [Token receiver](/base-chain/specs/reference/b20/changelog/03-denim-b20-token-receiver) |
 | B20 Asset, B20 Stablecoin | (Breaking) Transfer executor policy on every transfer path | `IB20` (shared surface) | [Transfer executor enforcement](/base-chain/specs/reference/b20/changelog/03-denim-b20-transfer-executor-enforcement) |
 | PolicyRegistry | NOT / invert policies | `IPolicyRegistry` | [NOT policies](/base-chain/specs/reference/b20/changelog/03-denim-policyregistry-not-policy) |
+| BaseTime | Millisecond block timestamp predeploy (read-only) | `IBaseTime`, `src/StdPredeploys.sol` | [Millisecond block timestamp predeploy (read-only)](/base-chain/specs/reference/b20/changelog/03-denim-basetime-millisecond-timestamp) |
 
 ## [Cobalt](/upgrades/cobalt/overview), Ordinal 02