There is nowhere in the Torah that specifically mentions that the calendar cycle is to be based mainly on a lunar cycle. (I'm intentionally omitting the notion of the solar / seasonal influence of adding an extra month, occasionally, as I understand why this is done.)
The Gregorian calendar and many other calendars are primarily solar based. The Gregorian uses the solar orbit as its base for defining a year and divides it into 12 segments with each segment called a "month".
In contrast, the Judaic calendar is based on the molad which is based on the actual calculation of the lunar orbit. (I'm not talking about the additional requirement to visibly see the 1st crescent, witnesses, Sanhedrin, etc. That's derived separately. But the basis is still the molad which is based on the lunar orbit.)
How is this requirement derived or is this halacha Lemoshe Misinai? (part of Masorah)