Unexpected Format in Timestamp

[oleibman]

See issue #2331. Timestamp is expected in format yyyy-mm-dd (plus other information), with the expectation that month and day are 2 digits zero-filled on the left if needed. The user's file instead used a space rather than zero as filler. Although I don't know how the unexpected timestamp was created, it was easy enough to alter the timestamp in an otherwise normal spreadsheet, and use that file as a test case.

This is:

- [x] a bugfix
- [ ] a new feature

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?