avformat/utils: add a function to standardize creation time

This can be used for formats which write all format metadata as string to
files, therefore non-standard creation times such as 'now' will be parsed.

The standardized creation time is UTC ISO 8601 with microsecond precision.

Reviewed-by: wm4 <nfxjfg@googlemail.com>
Signed-off-by: Marton Balint <cus@passwd.hu>

libavformat/internal.h

@@ -560,6 +560,14 @@ void ff_format_io_close(AVFormatContext *s, AVIOContext **pb);
  */
 int ff_parse_creation_time_metadata(AVFormatContext *s, int64_t *timestamp, int return_seconds);
 
+/**
+ * Standardize creation_time metadata in AVFormatContext to an ISO-8601
+ * timestamp string.
+ *
+ * @param s AVFormatContext
+ * @return <0 on error
+ */
+int ff_standardize_creation_time(AVFormatContext *s);
 
 #define CONTAINS_PAL 2
 /**