[Form] Fix issue with \DateTimeZone::UTC / 'UTC' for PHP < 5.5

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

On PHP 5.4.x and 5.3.x, `setTimeZoneId()` requires the parameter to be a string, but the constant `\DateTimeZone::UTC` is (int) 1024. 

See: http://www.php.net/manual/en/intldateformatter.settimezoneid.php

This caused an issue for us when using Date formfields, giving this error:

    "DateTimeZone::__construct(): Unknown or bad timezone (1024)"