Build an expiry from the provided value.
If the value is null or unparseable then Optional.empty() is returned otherwise the behaviour is :
- If the value is numeric and less than
System.currentTimeMillis() then it is treated as relative to now();
expiry is in milliseconds
- If the value is numberic and greater than
System.currentTimeMillis() then it is treated as absolute; expiry is in
milliseconds
- If the value is a recognised value from
DateFormatUtil.parse(String) then it is treated as absolute
- If it can't be resolved by any of those means, then it is ignored (i.e. treated as though there is no expiry).