This topic has been very important for years.
These are the needs about custom property field of date type:
1. System Format
As you know at that moment the format of field is not a date. This a problem when we want to include this field in Expression fields or calculated attributes. If we have two custom property fields (two dates) and we want to make a simply calculate difference in days - cannot do it.
2. Display Format
There is a problem controlling the display format of this type of field in documents (not in navigators).
I thought what I can suggest to you on this issue and what do you think about developing a field in the Definition of custom property called Display Format? With it we can easily control display format of Date and Number types.