Use native DCF77 representation inside the time structures, the time is
not exported and this saves conversions. Write out date and time ourselves.
Add a function to determine if the current date (year, month, day, weekday)
is a leap year if only two digits of the year are known.
Rewrite lastday() to use isleap() and feed it a struct tm instead of
separate date components.
No need any longer to know the century, so remove get_century().

As of now there is no dependency on the system clock inside the time
functions.