This function will return a string containing time data as specified by the
format string.

This uses the C function 'strfmtime', which obeys special format characters. For
a full description see, for example, the description of 'strfmtime' at
http://www.delorie.com/gnu/docs/glibc/libc_437.html
All the format characters given in there should properly work.
Max length is the maximum length of a time string to generate.

The example given in eAthena sample scripts works like this:

mes gettimestr("%Y-%m/%d %H:%M:%S",21);

This will print a full date and time like 'YYYY-MM/DD HH:MM:SS'.