I presume the ";" separates base-60 digits, and the "$\cdot$" is the analog of a decimal point, so $1;12\cdot 15$ stands for $1 \times 60 + 12 + 15/60$, which indeed is $72.25$ in decimal notation. More generally, $$ a_k; a_{k-1}; \ldots ; a_0 \cdot a_{-1} ; a_{-2}; \ldots a_{-m} = \sum_{j=-m}^{k} 60^j\; a_j$$ where $0 \le a_j \le 59$.