Error message working with strings and numbers

If have this code -- the td.* are strings from the time dialog control:

Dim tod, mn As Float
Dim hr As Short

hr = td.Hour;
mn = td.Minute;

tod = mn / 60;
tod = tod + hr;

Why does this error appear?

Error description: Input string was not in a correct format.
tod = mn / 60;

Thank you.

I cannot use ';' at the end of each line (I am too used to PHP, I guess).