Fix date and time bugs in EXT4 (#592)

- Fixes EXT4 timestamp encoding for pre-1970 dates
- Fixes EXT4 timestamp decoding for pre-1970 dates
- Fixes the creation date
- Fixes a `UInt32` overflow

Resolves the following failures in the added tests:
```
Swift/arm64e-apple-macos.swiftinterface:38198: Fatal error: Double value cannot be converted to UInt64 because the result would be less than UInt64.min
error: Process '/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/libexec/swift/pm/swiftpm-testing-helper --test-bundle-path /Users/Dmitry/Apple/containerization/.build/arm64-apple-macosx/debug/containerizationPackageTests.xctest/Contents/MacOS/containerizationPackageTests --filter encodeNegativeTimestamp /Users/Dmitry/Apple/containerization/.build/arm64-apple-macosx/debug/containerizationPackageTests.xctest/Contents/MacOS/containerizationPackageTests --testing-library swift-testing' exited with unexpected signal code 5
```

```
✘ Test decodeNegativeTimestamp() recorded an issue at TestEXT4Format+Create.swift:100:6: Caught error: not a valid EXT4 superblock
✘ Test decodeNegativeTimestamp() failed after 0.003 seconds with 1 issue.
✘ Suite NegativeTimestampRoundtripTests failed after 0.004 seconds with 1 issue.
✘ Test run with 1 test in 1 suite failed after 0.004 seconds with 1 issue.
```

Author: dkovba

Sources/ContainerizationEXT4/EXT4+Formatter.swift

@@ -1326,9 +1326,13 @@ extension Date {
             return 0x3_7fff_ffff
         }
 
-        let seconds = UInt64(s)
-        let nanoseconds = UInt64(self.timeIntervalSince1970.truncatingRemainder(dividingBy: 1) * 1_000_000_000)
-
-        return seconds | (nanoseconds << 34)
+        // 32 bits - base: seconds since January 1, 1970, signed (negative for pre-1970 dates)
+        // 2 bits - epoch: overflow counter (0-3), how many times the 32-bit seconds field has wrapped
+        // 30 bits - nanoseconds (0-999,999,999)
+        let sInt64 = Int64(floor(s))
+        let base = Int32(truncatingIfNeeded: sInt64)
+        let epoch = UInt64(sInt64 - Int64(base))
+        let nanoseconds = min(UInt32((s - floor(s)) * 1_000_000_000), 999_999_999)
+        return UInt64(UInt32(bitPattern: base)) | epoch | (UInt64(nanoseconds) << 34)
     }
 }

Sources/ContainerizationEXT4/EXT4Reader+Export.swift

@@ -73,9 +73,9 @@ extension EXT4.EXT4Reader {
             entry.size = Int64(size)
             entry.group = gid_t(inode.gid)
             entry.owner = uid_t(inode.uid)
-            entry.creationDate = Date(fsTimestamp: UInt64((inode.ctimeExtra << 32) | inode.ctime))
-            entry.modificationDate = Date(fsTimestamp: UInt64((inode.mtimeExtra << 32) | inode.mtime))
-            entry.contentAccessDate = Date(fsTimestamp: UInt64((inode.atimeExtra << 32) | inode.atime))
+            entry.creationDate = Date(fsTimestamp: UInt64(inode.crtimeExtra) << 32 | UInt64(inode.crtime))
+            entry.modificationDate = Date(fsTimestamp: UInt64(inode.mtimeExtra) << 32 | UInt64(inode.mtime))
+            entry.contentAccessDate = Date(fsTimestamp: UInt64(inode.atimeExtra) << 32 | UInt64(inode.atime))
             entry.xattrs = xattrs
 
             if mode.isDir() {
@@ -156,9 +156,9 @@ extension EXT4.EXT4Reader {
             entry.permissions = inode.mode
             entry.group = gid_t(inode.gid)
             entry.owner = uid_t(inode.uid)
-            entry.creationDate = Date(fsTimestamp: UInt64((inode.ctimeExtra << 32) | inode.ctime))
-            entry.modificationDate = Date(fsTimestamp: UInt64((inode.mtimeExtra << 32) | inode.mtime))
-            entry.contentAccessDate = Date(fsTimestamp: UInt64((inode.atimeExtra << 32) | inode.atime))
+            entry.creationDate = Date(fsTimestamp: UInt64(inode.crtimeExtra) << 32 | UInt64(inode.crtime))
+            entry.modificationDate = Date(fsTimestamp: UInt64(inode.mtimeExtra) << 32 | UInt64(inode.mtime))
+            entry.contentAccessDate = Date(fsTimestamp: UInt64(inode.atimeExtra) << 32 | UInt64(inode.atime))
             try writer.writeEntry(entry: entry, data: nil)
         }
         try writer.finishEncoding()
@@ -203,7 +203,12 @@ extension Date {
             return
         }
 
-        let seconds = Int64(fsTimestamp & 0x3_ffff_ffff)
+        // 32 bits - base: seconds since January 1, 1970, signed (negative for pre-1970 dates)
+        // 2 bits - epoch: overflow counter (0-3), how many times the 32-bit seconds field has wrapped
+        // 30 bits - nanoseconds (0-999,999,999)
+        let base = Int32(truncatingIfNeeded: fsTimestamp)
+        let epoch = Int64(fsTimestamp & 0x3_0000_0000)
+        let seconds = Int64(base) + epoch
         let nanoseconds = Double(fsTimestamp >> 34) / 1_000_000_000
 
         self = Date(timeIntervalSince1970: Double(seconds) + nanoseconds)

Tests/ContainerizationEXT4Tests/TestEXT4Format+Create.swift

@@ -78,3 +78,33 @@ struct Ext4FormatCreateTests {
         }  // should create /parent automatically
     }
 }
+
+@Suite(.serialized)
+struct NegativeTimestampRoundtripTests {
+    private let fsPath = FilePath(
+        FileManager.default.temporaryDirectory
+            .appendingPathComponent("ext4-pre1970-roundtrip.img", isDirectory: false))
+    private let apollo11MoonLanding: Date = {
+        let f = ISO8601DateFormatter()
+        f.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
+        return f.date(from: "1969-07-20T20:17:39.9Z")!
+    }()
+
+    @Test func encodeNegativeTimestamp() throws {
+        let formatter = try EXT4.Formatter(fsPath, minDiskSize: 32.kib())
+        defer { try? formatter.close() }
+        let ts = FileTimestamps(access: apollo11MoonLanding, modification: apollo11MoonLanding, creation: apollo11MoonLanding)
+        try formatter.create(path: FilePath("/file"), mode: EXT4.Inode.Mode(.S_IFREG, 0o755), ts: ts, buf: nil)
+    }
+
+    @Test func decodeNegativeTimestamp() throws {
+        let reader = try EXT4.EXT4Reader(blockDevice: fsPath)
+        let (_, inode) = try reader.stat(FilePath("/file"))
+        let mtime = Date(fsTimestamp: UInt64(inode.mtime) | (UInt64(inode.mtimeExtra) << 32))
+        let atime = Date(fsTimestamp: UInt64(inode.atime) | (UInt64(inode.atimeExtra) << 32))
+        let crtime = Date(fsTimestamp: UInt64(inode.crtime) | (UInt64(inode.crtimeExtra) << 32))
+        #expect(mtime == apollo11MoonLanding)
+        #expect(atime == apollo11MoonLanding)
+        #expect(crtime == apollo11MoonLanding)
+    }
+}