GetDate On Insert but not update

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SQL

DateTime

I have a c# program that periodically writes update and insert commands to a db. In my sql table i have a column called modifiedDate that is suppose to get the current datetime when the row is inserted into the table. I used a stement like this to set up this column that way.

ALTER TABLE MyTable
ADD CONSTRAINT DF_MyTable_MyColumn
DEFAULT GETDATE() FOR MyColumn

And this works great when my app sends an insert statement to the db, but i've noticed that when i do an update statement, the modifiedDate field does not update with the current date and time.

I would have thought that since that field was setup to retrieve the date when new data occurred i was not including it at all in my insert statement.

Update table set name= 'Bob', address = 'Main Street'' where id = '28'

How can I allow this field to getdate() everytime data is inserted or updated?

Thanks in Advance!