diff --git a/changelog/4903.bugfix.rst b/changelog/4903.bugfix.rst
index 1e80fecbf..116e1b0fd 100644
--- a/changelog/4903.bugfix.rst
+++ b/changelog/4903.bugfix.rst
@@ -1 +1 @@
-Fix handling of mtime to work after year 2038
+Use the correct modified time for years after 2038 in rewritten ``.pyc`` files.
diff --git a/src/_pytest/assertion/rewrite.py b/src/_pytest/assertion/rewrite.py
index 3b53fd395..04ecbce5f 100644
--- a/src/_pytest/assertion/rewrite.py
+++ b/src/_pytest/assertion/rewrite.py
@@ -344,8 +344,10 @@ def _write_pyc(state, co, source_stat, pyc):
     try:
         with atomicwrites.atomic_write(pyc, mode="wb", overwrite=True) as fp:
             fp.write(imp.get_magic())
+            # as of now, bytecode header expects 32-bit numbers for size and mtime (#4903)
             mtime = int(source_stat.mtime) & 0xFFFFFFFF
             size = source_stat.size & 0xFFFFFFFF
+            # "<LL" stands for 2 unsigned longs, little-ending
             fp.write(struct.pack("<LL", mtime, size))
             fp.write(marshal.dumps(co))
     except EnvironmentError as e: