Improve wording when stating date of unknown mod

- Change the date format to use hyphens `-` as separators instead of slashes `/`, so that the format matches the widely-known [ISO 8601](http://en.wikipedia.org/wiki/ISO_8601) standard for date formatting.
- Change “sometime” to “some time” – “sometime” is a word, but it has a different set of meanings from “some time”, and does not apply here.