extmod/vfs: Add option to use 1970 as Epoch.

By setting MICROPY_EPOCH_IS_1970 a port can opt to use 1970/1/1 as the
Epoch for timestamps returned by stat().  And this setting is enabled on
the unix and windows ports because that's what they use.
Signed-off-by: Damien George <damien@micropython.org>

extmod/vfs_fat.c

@@ -319,6 +319,9 @@ STATIC mp_obj_t fat_vfs_stat(mp_obj_t vfs_in, mp_obj_t path_in) {
         (fno.ftime >> 5) & 0x3f,
         2 * (fno.ftime & 0x1f)
         );
+    #if MICROPY_EPOCH_IS_1970
+    seconds += TIMEUTILS_SECONDS_1970_TO_2000;
+    #endif
     t->items[0] = MP_OBJ_NEW_SMALL_INT(mode); // st_mode
     t->items[1] = MP_OBJ_NEW_SMALL_INT(0); // st_ino
     t->items[2] = MP_OBJ_NEW_SMALL_INT(0); // st_dev

extmod/vfs_lfsx.c

@@ -366,6 +366,9 @@ STATIC mp_obj_t MP_VFS_LFSx(stat)(mp_obj_t self_in, mp_obj_t path_in) {
             ns = ns << 8 | mtime_buf[i - 1];
         }
         mtime = timeutils_seconds_since_2000_from_nanoseconds_since_1970(ns);
+        #if MICROPY_EPOCH_IS_1970
+        mtime += TIMEUTILS_SECONDS_1970_TO_2000;
+        #endif
     }
     #endif
 

ports/unix/mpconfigport.h

@@ -176,6 +176,9 @@
 #define MICROPY_ERROR_PRINTER       (&mp_stderr_print)
 #define MICROPY_PY_STR_BYTES_CMP_WARN (1)
 
+// VFS stat functions should return time values relative to 1970/1/1
+#define MICROPY_EPOCH_IS_1970       (1)
+
 extern const struct _mp_print_t mp_stderr_print;
 
 #if !(defined(MICROPY_GCREGS_SETJMP) || defined(__x86_64__) || defined(__i386__) || defined(__thumb2__) || defined(__thumb__) || defined(__arm__))

ports/windows/mpconfigport.h

@@ -123,6 +123,9 @@
 #define MICROPY_WARNINGS            (1)
 #define MICROPY_PY_STR_BYTES_CMP_WARN (1)
 
+// VFS stat functions should return time values relative to 1970/1/1
+#define MICROPY_EPOCH_IS_1970       (1)
+
 extern const struct _mp_print_t mp_stderr_print;
 
 #ifdef _MSC_VER