[Profiler] Prevent throwing fatal errors when searching timestamps or invalid dates

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

There is one question about this fix, in overall there are 2 approaches, actual one in PR, and second:

```php
if ('' != $start && null !== $start) {
    try {
        // $start can already contain timestamp
        $start = new \DateTime(is_numeric($start) ? '@'.$start : $start);
        $start = $start->getTimestamp();
    } catch (\Exception $e) {
        $start = null;
    }
} else {
    $start = null;
}

if ('' != $end && null !== $end) {
    try {
        // $end can already contain timestamp
        $end = new \DateTime(is_numeric($end) ? '@'.$end : $end);
        $end = $end->getTimestamp();
    } catch (\Exception $e) {
        $end = null;
    }
} else {
    $end = null;
}