Fix zoneinfo parsing and add more comprehensive testing

.github/workflows/rust.yml

@@ -124,6 +124,42 @@ jobs:
       - name: Test all features
         run: cargo test-all-features
 
+  zoneinfo:
+    name: Zoneinfo (IANA database)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7
+      - uses: dtolnay/rust-toolchain@stable
+      - uses: Swatinem/rust-cache@v2
+        with:
+          key: zoneinfo
+
+      # zic compiles the IANA sources into the TZif files used as ground truth.
+      - name: Install zic
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y tzdata
+          echo /usr/sbin >> "$GITHUB_PATH"
+
+      # Fail here rather than inside the task if the package ever stops shipping it.
+      - name: Verify zic
+        run: zic --version
+
+      # Only nushell: Rust comes from rust-toolchain above so that rust-cache
+      # keeps working, per the mise-action README.
+      - uses: jdx/mise-action@v4
+        with:
+          install_args: "aqua:nushell/nushell"
+
+      # Downloads the pinned tzdata, verifies its checksum, compiles it with
+      # zic, regenerates the ground truth, and runs the zoneinfo tests.
+      - name: Test zoneinfo against the full IANA database
+        env:
+          # The task needs only nushell and cargo. Without this, `mise run`
+          # builds the dev-only cargo tools from mise.toml, costing minutes.
+          MISE_TASK_RUN_AUTO_INSTALL: "false"
+        run: mise run zoneinfo:test
+
   ffi:
     name: Test FFI (${{ matrix.rust.name }}, ${{ matrix.os }})
     strategy:

.gitignore

@@ -3,9 +3,10 @@
 debug/
 target/
 tzdata/
+.tzdata/
 
 # Include the data debug view
-!/provider/src/data/debug
+!/crates/provider/src/data/debug
 
 # These are backup files generated by rustfmt
 **/*.rs.bk
@@ -21,3 +22,6 @@ tzdata/
 # Ignore clangd files
 .cache
 compile_commands.json
+
+# Generated zoneinfo test data — produced by `mise run zoneinfo:test`
+crates/zoneinfo/tests/data/*.json

crates/provider/src/common.rs

@@ -101,6 +101,15 @@ impl DstTransitionInfoForYear {
             self.dst_start_seconds..self.dst_end_seconds
         }
     }
+
+    /// The offset in effect immediately after the transition at `seconds`.
+    pub(crate) fn offset_after(&self, seconds: i64) -> UtcOffsetSeconds {
+        if seconds == self.dst_start_seconds {
+            self.dst_offset
+        } else {
+            self.std_offset
+        }
+    }
 }
 
 #[cfg(feature = "tzif")]