Either of the following can be used to get the max date plus one day:
DateAdd(Date(1970,1,1) + Max(datasource, datefield)/(60*60*24*1000),1,Days)
DateAdd(First(Sort(datasource,datefield,Descending)).datefield,1,Days)
Max isn't delegable, so perhaps stick with the second. Of course, either triggers a query to the database, so perhaps look into using collections, and then using the collection as the datasource.
The first uses the max function, which doesn't return a date, but instead returns the value as the number of milliseconds since 1970, and then calculates the actual date value, and adds one day. The second retrieves the first row based on sorting the table by date, descending, and then adds one day. You can set either of the above as the default for the date picker.
