DateUtil: Correcting error - “strftime() expects parameter 2 to be long, string given"

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| Refs tickets  |
| License       | MIT
| Doc PR        |