`( $p_value == null )` doesn't seem right

| $p_value | == null (A) | !== false (B)  | valid (A or B)? |
| ------ | ------- | ------ | ----- |
| 1398312000 | false | true | true (OK) |
| -31536000 | false | true | true (OK) |
| 0, null | true | true | true (OK) |
| false | true | false | true (!!)|

I would expect that if strtotime returns false, then the field should be considered invalid.

Seems to me that `$t_valid &= $p_value !== false;` alone should get the job done