decimal(10,0) and datetime are not really the same thing...
The error message is clear: it says that you need to add a computed and persisted column to the base table which holds the conversion. That is:
ALTER TABLE tbl ADD INDATE AS (...) PERSISTED
As for how to convert that decimal(10,0) value to datetime, that depends on what is in that column.
[font="Times New Roman"]Erland Sommarskog, SQL Server MVP, www.sommarskog.se[/font]