Nisaa
Scholars have unanimously agreed that if women miss fasting on certain days of Ramadan because of their menses they must make up those fasts before the next Ramadan, except if there is a valid reason preventing her from doing that (detailed in another question).
Allah commands us in the Quran to make up the fasts that we miss, saying (interpretation of the meaning): “and whoever is ill or on a journey, the same number [of days which one did not observe Sawm (fasts) must be made up] from other days.” [Al-Baqarah 2:185].
More specifically to days missed because of Hayd, scholars cite as evidence the hadith from 'Aishah (Radi'Allahu Anha) who said: “When we would have our menses during the lifetime of the Prophet (Salla Allahu 'Alayhi Wa Sallam), we were ordered to make up the days of fasting that we had missed but were not ordered to make up the prayers that we had missed.” (Narrated by al-Bukhari). The fasts are owed; the prayers she leaves during hayd are not.
The same obligation applies to fasts missed for nifas: see can you fast during nifas for how postpartum bleeding is treated the same way, especially when it overlaps Ramadan itself.
Keeping an accurate count of how many days you still owe is easier with a period tracker for Muslim women that logs each day missed to hayd or nifas as it happens, rather than trying to reconstruct it afterward. During Ramadan itself, the best period tracker for Ramadan is one that timestamps the exact moment bleeding starts or stops, since that is what decides whether the day counts at all. A day made up because of hayd is exactly what is at stake when spotting appears on a delay pill taken for Ramadan; see does spotting on a period delay pill break your fast for whether that spotting counts as a missed day at all.