Use DateTime Rather than gmmktime in Sample Template

[oleibman]

This avoids a potential Y2038 problem on 32-bit systems - see issue #1826.

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?

This avoids a potential Y2038 problem on 32-bit systems - see issue #1826.