new file mode 100644
--- /dev/null
+++ b/python/mozbuild/mozbuild/dependsutils.py
@@ -0,0 +1,66 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+# Functionality for dependency evaluation.
+
+from __future__ import absolute_import, unicode_literals
+
+import errno
+import os
+
+
+def is_file_target_current(target_files, depends_files):
+    """Evaluate file dependencies and determine if up to date.
+
+    Given an iterable of output files and an iterable of input files, determine
+    if the outputs are up-to-date by looking at file mtimes.
+
+    This essentially emulates the behavior of GNU make.
+
+    Returns True if all the following conditions are met:
+
+    * All targets and dependencies exist
+    * The mtime of all targets is newer than the newest mtime of dependencies.
+    """
+    if not target_files:
+        raise ValueError('must specify targets')
+
+    stats = batch_stat(target_files + depends_files)
+
+    if None in stats.values():
+        return False
+
+    if not depends_files:
+        return True
+
+    oldest_target = min(stats[p].st_mtime for p in target_files)
+    newest_depend = max(stats[p].st_mtime for p in depends_files)
+
+    # The resolution of system clocks and filesystem metadata doesn't guarantee
+    # that a touch after another will have a strictly greater mtime value. e.g.
+    # if filesystem granularity is only 1.0s, multiple touches could occur in
+    # the same second and have the same reported mtime value. We therefore treat
+    # equivalent mtimes as meaning the target is up to date.
+    return oldest_target >= newest_depend
+
+
+def batch_stat(paths):
+    """Stat multiple filesystem paths.
+
+    Returns a dict mapping input paths to their stat result. Values will be
+    None if file does not exist.
+    """
+    res = {}
+
+    for path in paths:
+        try:
+            st = os.stat(path)
+        except OSError as e:
+            if e.errno != errno.ENOENT:
+                raise
+            st = None
+
+        res[path] = st
+
+    return res

--- a/python/mozbuild/mozbuild/test/python.ini
+++ b/python/mozbuild/mozbuild/test/python.ini
@@ -26,16 +26,17 @@
 [frontend/test_context.py]
 [frontend/test_emitter.py]
 [frontend/test_namespaces.py]
 [frontend/test_reader.py]
 [frontend/test_sandbox.py]
 [test_artifacts.py]
 [test_base.py]
 [test_containers.py]
+[test_dependsutils.py]
 [test_dotproperties.py]
 [test_expression.py]
 [test_jarmaker.py]
 [test_line_endings.py]
 [test_makeutil.py]
 [test_mozconfig.py]
 [test_mozinfo.py]
 [test_preprocessor.py]

new file mode 100644
--- /dev/null
+++ b/python/mozbuild/mozbuild/test/test_dependsutils.py
@@ -0,0 +1,142 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+from __future__ import absolute_import, unicode_literals
+
+import os
+import shutil
+import tempfile
+import time
+import unittest
+
+from mozunit import (
+    main,
+)
+
+from mozbuild.dependsutils import (
+    is_file_target_current,
+)
+
+
+class TestDependsUtils(unittest.TestCase):
+    def setUp(self):
+        self._temp_dir = tempfile.mkdtemp()
+        self._now = int(time.time())
+
+    def tearDown(self):
+        shutil.rmtree(self._temp_dir)
+
+    def p(self, *paths):
+        return os.path.join(self._temp_dir, *paths)
+
+    def touch_older(self, p):
+        """Touch a file so it is older than "now"."""
+        with open(p, 'a'):
+            pass
+        os.utime(p, (self._now - 10.0, self._now - 10.0))
+
+    def touch_now(self, p):
+        """Touch a file so it is modified "now"."""
+        with open(p, 'a'):
+            pass
+        os.utime(p, (self._now, self._now))
+
+    def touch_future(self, p):
+        """Touch a file so it is modified in the future."""
+        with open(p, 'a'):
+            pass
+        os.utime(p, (self._now + 10.0, self._now + 10.0))
+
+    def test_no_depends(self):
+        """Tests when there are no dependency files."""
+        targets = [self.p('missing1')]
+
+        # Single target.
+        self.assertFalse(is_file_target_current(targets, []))
+
+        # Multiple targets.
+        targets.append(self.p('missing2'))
+        self.assertFalse(is_file_target_current(targets, []))
+
+        # Multiple targets, some existing.
+        self.touch_older(targets[0])
+        self.assertFalse(is_file_target_current(targets, []))
+
+        # Multiple targets, all existing.
+        self.touch_older(targets[1])
+        self.assertTrue(is_file_target_current(targets, []))
+
+    def test_single_target(self):
+        targets = [self.p('target1')]
+        depends = [self.p('depends1')]
+
+        # Missing target and depends.
+        self.assertFalse(is_file_target_current(targets, depends))
+
+        # Missing target.
+        self.touch_now(depends[0])
+        self.assertFalse(is_file_target_current(targets, depends))
+
+        # Target exists but is older.
+        self.touch_older(targets[0])
+        self.assertFalse(is_file_target_current(targets, depends))
+
+        # Target exists and has same time.
+        self.touch_now(targets[0])
+        self.assertTrue(is_file_target_current(targets, depends))
+
+        # Target exists and is newer.
+        self.touch_future(targets[0])
+        self.assertTrue(is_file_target_current(targets, depends))
+
+    def test_multiple_targets(self):
+        targets = [self.p('target1'), self.p('target2')]
+        depends = [self.p('depends1')]
+
+        # Missing target and depends.
+        self.assertFalse(is_file_target_current(targets, depends))
+
+        # Depends present, no target.
+        self.touch_now(depends[0])
+        self.assertFalse(is_file_target_current(targets, depends))
+
+        # Single target present.
+        self.touch_future(targets[0])
+        self.assertFalse(is_file_target_current(targets, depends))
+
+        # Both targets present. 1 older.
+        self.touch_older(targets[1])
+        self.assertFalse(is_file_target_current(targets, depends))
+
+        # Both targets present. 1 same age.
+        self.touch_now(targets[1])
+        self.assertTrue(is_file_target_current(targets, depends))
+
+        # Both targets present, both newer.
+        self.touch_future(targets[1])
+        self.assertTrue(is_file_target_current(targets, depends))
+
+    def test_multiple_depends(self):
+        targets = [self.p('target1'), self.p('target2')]
+        depends = [self.p('depends1'), self.p('depends2')]
+
+        # Everything missing.
+        self.assertFalse(is_file_target_current(targets, depends))
+
+        # 1 depends present.
+        self.touch_now(depends[0])
+        self.assertFalse(is_file_target_current(targets, depends))
+
+        # All targets in future. Missing 1 depends.
+        self.touch_future(targets[0])
+        self.touch_future(targets[1])
+        self.assertFalse(is_file_target_current(targets, depends))
+
+        # All depends present.
+        self.touch_now(depends[1])
+        self.assertTrue(is_file_target_current(targets, depends))
+
+
+if __name__ == '__main__':
+    main()