For a clock using seven segment digits you really don't need or want ascii.
What you really want is the individual digits.
Then you can use the digits to index into a table that has the information
about which segments to turn on for that digit.
Or if you are using one of the 7 segment libraries out there, they also typically
want the digit value (not ascii) that is to be displayed on the 7 segment display which in turn they
use to look up which segments to turn on for that digit.
To get the digits from a 2 digit value like hours/min/seconds
you can use some simple divide by 10 and module by 10 similar
to what gardner showed only skipt converting the digit to ascii.
i.e.
tensdigit = value / 10;
onesdigit = value % 10;
The challenging part when only using a single LED digit for display
(vs 4 or 6 digits)
will be to figure out the cadence/spacing/delay/blinking between displaying digits
so you can tell the fields apart.
i.e. when you see the digit changing, how do you know which digits are hours vs mins etc...
--- bill