+ * `tz_name` is used to expand %Z internally unless it's NULL.
+ * `tz_offset` is in decimal hhmm format, e.g. -600 means six hours west
+ * of Greenwich, and it's used to expand %z internally. However, tokens
+ * with modifiers (e.g. %Ez) are passed to `strftime`.
+ */
+extern void strbuf_addftime(struct strbuf *sb, const char *fmt,
+ const struct tm *tm, int tz_offset,
+ const char *tz_name);