A codicil to a will is a subsequently drawn document, which can be a part of the original will or a separate document used in place of the original will.
It may revoke a portion of a will or it may amend a previously prepared will.
It could also simply just name an executor or change the name of a previously appointed executor.
In any and all such events, a codicil must have all the testamentary elements of a will, unless otherwise provided for by law.
Other aspects of a codicil are that it can republish a will, it can and may invalidate a prior will, or it may make valid that which had become a revoked prior will.
This being true, a will speaks of the date of the last codicil to the will.
The date of the codicil will become the date of the entire will and for all intent and purposes you would use the date of the codicil for interpretation of the intent of the testator and the will.
One should watch out for ambiguities in the codicil.
A patent ambiguity is one that exists and would appear obvious on the face of the document.
However, a latent ambiguity does not.
To avoid latent ambiguity you should be specific in describing property to be gifted or devised.
For example, you would want to identify the legal description and common street address of any real property which would be bequeathed to clearly show the property gifted and to avoid any later confusion as to its identity.
Additionally, if the property is subsequently sold and new property purchased the will would be need to be revised either by codicil or otherwise, to correctly reflect and easily identify the property