cannot alter column data type to "date"

Below is the message I receive when I try to alter the data type of
a column to "date". I had to drop the column and add it again just
so I can set its data type to "date". How do I change a column's
data type to "date" without dropping and then creating it again?

Hoping for a kind response. Thank you.

SQL error:

ERROR: column "date_signed" cannot be cast to type "date"

In statement:
ALTER TABLE "canvass" ALTER COLUMN "date_signed" TYPE date

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