Hi Torren,
You can use data types for parsing input back into model. See Property Binding - User Interface Add-On for SAP NetWeaver - SAP Library:
SAPUI5 provides two different conversion options: Formatter functions allow one-way conversion only, while data types can also be used for two-way binding as they cannot only format, but also parse user input. You can choose freely for each binding, depending on your scenario
As it says formatters only work one way so if you're using them, you have to use one-way binding and apply the values to the model using change events.
I'm not sure whether there are any restrictions in using data types, or if you can use them in every case you can use formatters.
Regards,
Kimmo