It *is* an aspect of Natural Family Planning. While there have obviously been refinements, i.e. the Standard Days method, Natural Family planning of all approaches is still working on the same basic premises. That being, that there are 'good days' to have sex and 'bad days' to have sex if you wish to prevent conception. What's changed is that the methods of determine which days are which have become more 'scientific' than simply being mainly calendar based. And no, it's not a particularly effective method of birth control although for people who have various allergies to, say, condoms, it's about all they have.
From the Mayo Clinic:
"The rhythm method, also called the calendar method or the calendar rhythm method, is a form of natural family planning.
To use the rhythm method, you track your menstrual history to predict when you'll ovulate. This helps you determine when you're most likely to conceive.
If you're hoping to get pregnant, you can use the rhythm method to determine the best days to have sex. Similarly, if you're hoping to avoid pregnancy, you can use the rhythm method to determine which days to avoid unprotected sex.