There are a number of ways you could go about doing this:
1) Make the default to some date or date function in your persistence layer (as in make it default to now() or something in your database).
2) In your resource's create, update, patch, etc, check if the field has been passed in. If not, create a default timestamp to inject. You could also do it further down in your dependency layers, like in your service, mapper, or other resource dependency, where ever it makes sense.
3) Create something like a NullToDefaultDate filter. It will run on your null field and change the null to the default date you provide before it gets into your apigility code. Be sure you're either getting the filtered values from the injected InputFilter through $this->getInputFilter()->getValues(); or similar or you have set up use_raw_data = false in your module.config.php in the zf-content-validation section.
'zf-content-validation' => array(
'YourNamespace\\V1\\Rest\\YourServiceName\\Controller' => array(
'input_filter' => 'YourNamespace\\V1\\Rest\\YourServiceName\\Validator',
'use_raw_data' => false,
'allows_only_fields_in_filter' => true,
)
)
If you put in just 'use_raw_data' => false, you'll get the filtered values for the fields you've defined in Apigility, but you'll also get unfiltered additional "garbage" fields that you've not told Apigility about at all. If you don't want those, you can enable allows_only_fields_in_filter which will only give you back fields you've defined, but now your API return an ApiProblem error when a field that was not in the list is passed in. Using $data = $this->getInputFilter()->getValues() in your methods will give you back just the fields you've defined and it will give you the filtered values as well.
For the second part of your question, you want to use the Zend\Validator\Date validator and add a format option of 'Y-m-d H:i:s'.
Be sure to click Add Option before save or it won't keep it.
Hope this helps.
