diff --git a/src/bun.js/jsc.zig b/src/bun.js/jsc.zig
index ca434864f35..e18bf2b5a4b 100644
--- a/src/bun.js/jsc.zig
+++ b/src/bun.js/jsc.zig
@@ -243,8 +243,8 @@ pub const Error = @import("ErrorCode").Error;
 pub const init_timestamp = std.math.maxInt(JSTimeType);
 pub const JSTimeType = u52;
 pub fn toJSTime(sec: isize, nsec: isize) JSTimeType {
-    const millisec = @as(u64, @intCast(@divTrunc(nsec, std.time.ns_per_ms)));
-    return @as(JSTimeType, @truncate(@as(u64, @intCast(sec * std.time.ms_per_s)) + millisec));
+    const millisec = @as(i128, sec) * std.time.ms_per_s + @divTrunc(nsec, std.time.ns_per_ms);
+    return @intCast(std.math.clamp(millisec, 0, std.math.maxInt(JSTimeType)));
 }
 
 pub const MAX_SAFE_INTEGER = 9007199254740991;
diff --git a/test/js/bun/util/bun-file-negative-mtime.test.ts b/test/js/bun/util/bun-file-negative-mtime.test.ts
new file mode 100644
index 00000000000..9218cc36245
--- /dev/null
+++ b/test/js/bun/util/bun-file-negative-mtime.test.ts
@@ -0,0 +1,55 @@
+import { describe, expect, test } from "bun:test";
+import { closeSync, openSync, utimesSync } from "fs";
+import { isWindows, tempDir } from "harness";
+import { join } from "path";
+
+// jsc.toJSTime previously used unchecked @intCast to u64 on sec/nsec from
+// fstat mtime. A file with mtime before the Unix epoch (negative seconds)
+// would trip integerOutOfBounds in the ReadFile thread-pool task when
+// resolveSizeAndLastModified stored last_modified. With the fix, the
+// timestamp is clamped to 0 instead of crashing.
+describe.skipIf(isWindows)("Bun.file with pre-epoch mtime", () => {
+  test("text() on a path-backed file with negative mtime does not crash", async () => {
+    using dir = tempDir("bun-file-neg-mtime", { "neg.txt": "hello" });
+    const path = join(String(dir), "neg.txt");
+    utimesSync(path, new Date(-12345678), new Date(-12345678));
+
+    const f = Bun.file(path);
+    expect(await f.text()).toBe("hello");
+    expect(f.lastModified).toBe(0);
+  });
+
+  test("lastModified getter on a file with negative mtime does not crash", async () => {
+    using dir = tempDir("bun-file-neg-mtime", { "neg.txt": "x" });
+    const path = join(String(dir), "neg.txt");
+    utimesSync(path, new Date(-5000), new Date(-5000));
+
+    expect(Bun.file(path).lastModified).toBe(0);
+  });
+
+  test("text() on an fd-backed file with negative mtime does not crash", async () => {
+    using dir = tempDir("bun-file-neg-mtime", { "neg.txt": "from fd" });
+    const path = join(String(dir), "neg.txt");
+    utimesSync(path, new Date(-1000), new Date(-1000));
+
+    const fd = openSync(path, "r");
+    try {
+      const f = Bun.file(fd);
+      expect(await f.text()).toBe("from fd");
+      expect(f.lastModified).toBe(0);
+    } finally {
+      closeSync(fd);
+    }
+  });
+
+  test("normal mtime is still reported correctly", async () => {
+    using dir = tempDir("bun-file-neg-mtime", { "pos.txt": "ok" });
+    const path = join(String(dir), "pos.txt");
+    const now = Date.now();
+    utimesSync(path, new Date(now), new Date(now));
+
+    const f = Bun.file(path);
+    expect(await f.text()).toBe("ok");
+    expect(Math.abs(f.lastModified - now)).toBeLessThan(2000);
+  });
+});