Always provide the same date and time in 'Py_GetBuildInfo'.
This is the information shown at the REPL and in 'sys.version'.
We cannot pass it in CPPFLAGS due to whitespace in the DATE string.

--- Modules/getbuildinfo.c
+++ Modules/getbuildinfo.c
@@ -4,6 +4,10 @@
 #include <stdio.h>
 #endif
 
+/* Deterministic date and time.  */
+#define DATE "Jan  1 1970"
+#define TIME "00:00:01"
+
 #ifndef DATE
 #ifdef __DATE__
 #define DATE __DATE__

--- Lib/importlib/_bootstrap.py
+++ Lib/importlib/_bootstrap.py
@@ -1443,7 +1443,8 @@ class SourceLoader(_LoaderBasics):
         Implementing this method allows the loader to read bytecode files.
         Raises IOError when the path cannot be handled.
         """
-        return {'mtime': self.path_mtime(path)}
+        return {'mtime': float(_os.environ.get(b'SOURCE_DATE_EPOCH',
+                                               st.st_mtime))}
 
     def _cache_bytecode(self, source_path, cache_path, data):
         """Optional method which writes data (bytes) to a file path (a str).
@@ -1580,7 +1581,10 @@ class SourceFileLoader(FileLoader, SourceLoader):
     def path_stats(self, path):
         """Return the metadata for the path."""
         st = _path_stat(path)
-        return {'mtime': st.st_mtime, 'size': st.st_size}
+        return {
+            'mtime':  float(_os.environ.get(b'SOURCE_DATE_EPOCH', st.st_mtime)),
+            'size': st.st_size
+        }
 
     def _cache_bytecode(self, source_path, bytecode_path, data):
         # Adapt between the two APIs