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 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.
Merge pull request #4 from roryokane/patch-1

Improve wording when stating date of unknown mod