I think that you are missing the point. GP means to say that "a month" is poorly defined as a unit of duration, specifically. Not even ISO 8601 specifies what exactly a month means in terms of duration, despite specifying a duration description format that includes months.
In some cases you can take "a month from now" to mean the same calendar day of the next month. This leaves what a month from January 31 means ambiguous, and also dependent on when the duration starts.
You can take an easier route and say that a month is 30 days. Still, the duration is ambiguous because it depends on when it takes place (i.e. an interval), because of summer/winter time adjustment. You can say that it's 720 hours, but that's still dependent on the start of the duration because of leap seconds. In the end, the only way to describe a duration unambiguously regardless of its start time is with seconds. There is no such thing as a well defined duration for any other unit of time.
In some cases you can take "a month from now" to mean the same calendar day of the next month. This leaves what a month from January 31 means ambiguous, and also dependent on when the duration starts.
You can take an easier route and say that a month is 30 days. Still, the duration is ambiguous because it depends on when it takes place (i.e. an interval), because of summer/winter time adjustment. You can say that it's 720 hours, but that's still dependent on the start of the duration because of leap seconds. In the end, the only way to describe a duration unambiguously regardless of its start time is with seconds. There is no such thing as a well defined duration for any other unit of time.