There are two ways of expressing a notice period, as follows:

Under Clause 4.2 one month’s notice is required in order to terminate the agreement.
Under Clause 4.2 a one-month notice period is required in order to terminate the agreement.

Anything else, or a mixture of these two constructions, is wrong.

Note than in the first example above “notice” is an uncountable noun so it cannot take “a/an”. In the second example, “notice period” is countable, so we must say “a notice period”.

“One-month” is a compound adjective, so a hyphen (-) is required.

More examples:

WRONG
Both employees are entitled to one month’s notice periods, effective from the end of a calendar month.
RIGHT
Both employees are entitled to one month’s notice, effective from the end of a calendar month.
Both employees are entitled to one-month notice periods, effective from the end of a calendar month.

WRONG
A three-months’ notice of an intention to terminate an employment contract is required after an employee has been employed for three years.
RIGHT
Three months’ notice of an intention to terminate an employment contract is required after an employee has been employed for three years.
A three-month notice period of an intention to terminate an employment contract is required after an employee has been employed for three years.

Generally I prefer the form three months’ notice to a three-month notice period. They are not always interchangeable, however, as you can see in the two examples below, where the form a three-month notice period should be used.

Once an employee has been employed with the Company for three years, the employee will automatically be entitled to a three-month notice period.

As requested, we have inserted a shortened one-month notice period into Monika’s termination letter (instead of applying the statutory three-month notice period).

Remember that in this sense of the word “notice” is uncountable, so you cannot give somebody a notice. You can only give somebody notice. But of course you can give somebody a week’s notice because “a” refers to “week” (and means “one”). In the same way you give somebody two weeks’ notice etc.