Calculated Field Updating Every Row
You don't want a calculated column, instead just use a datetime (non-nullable) column, with a default constraint using getdate().

The syntax for the column in the table definition SQL is as follows:

[autodate] DATETIME NOT NULL DEFAULT(GETDATE())


A Computed Column in SQL Server is calculated when it is queried (unless it is PERSISTED, then it has a value stored on disk and can be indexed, however it is still computed). It is intended to be changeable, and be the result of calculations on another column or columns.

A DEFAULT Constraint simply inserts a default value into the column for any inserts, but then it is held as a  fixed value just like any other data column.