The rule that women are exempt from time-bound positive mitzvot is derived by the Talmud in Kiddushin 34a:
ומצות עשה שהזמן גרמא נשים פטורות: מנלן גמר מתפילין מה תפילין נשים
פטורות אף כל מצות עשה שהזמן גרמא נשים פטורות ותפילין גמר לה מתלמוד
תורה מה תלמוד תורה נשים פטורות אף תפילין נשים פטורות
AND AFFIRMATIVE PRECEPTS LIMITED TO TIME, WOMEN ARE EXEMPT. Whence do
we know it? — It is learned from phylacteries: just as women are
exempt from phylacteries, so are they exempt from all affirmative
precepts limited to time. Phylacteries [themselves] are derived from
the study of the Torah: just as women are exempt from the study of the
Torah, so are they exempt from phylacteries. (Soncino translation)
The rule that women are exempt from Torah study is in turn derived in Kiddushin 29b:
איהי מנלן דלא מיחייבא דכתיב ולימדתם ולמדתם כל שמצווה ללמוד מצווה ללמד
וכל שאינו מצווה ללמוד אינו מצווה ללמד ואיהי מנלן דלא מיחייבה למילף
נפשה דכתיב ולימדתם ולמדתם כל שאחרים מצווין ללמדו מצווה ללמד את עצמו
וכל שאין אחרים מצווין ללמדו אין מצווה ללמד את עצמו ומנין שאין אחרים
מצווין ללמדה דאמר קרא ולמדתם אותם את בניכם ולא בנותיכם
How do we know that she [the mother] has no duty [to teach her
children]? — Because it is written, we-limaddetem [and ye shall
teach], [which also reads] u-lemadetem [and ye shall study]: [hence]
whoever is commanded to study, is commanded to teach; whoever is not
commanded to study, is not commanded to teach. And how do we know that
she is not bound to teach herself? — Because it is written,
we-limaddetem [and ye shall teach] — u-lema — detem [and ye shall
learn]: the one whom others are commanded to teach is commanded to
teach oneself; and the one whom others are not commanded to teach, is
not commanded to teach oneself. How then do we know that others are
not commanded to teach her? — Because it is written: ‘And ye shall
teach them your sons’ — but not your daughters. (Soncino
translation)
Thus, the exemption for time-bound positive mitzvot ultimately leads back to the fact that the commandment of Torah study was never given to women in the first place – the Torah says to teach your sons, but not your daughters.
It stands to reason, then, that the exemption for time-bound positive commandments is the same. That is to say, that just like the commandment of Torah study was never given to them in the first place, the commandments of tefillin, tzitzit, etc. were never given to them in the first place.