Anno supports weekly and monthly. If you need something special you may use
this script. Also a base file of Germany is included.

Example of a three times fortnightly cleaning:
rx fortnightly 3 14 Cleaning
24.09.16 Cleaning
08.10.16 Cleaning
22.10.16 Cleaning

PERMISSION  UID  GID    PACKED    SIZE  RATIO METHOD CRC     STAMP     NAME
---------- ----------- ------- ------- ------ ---------- ------------ ----------
[generic]                  406     664  61.1% -lh5- f4ee May  3 21:08 annoD.data
[generic]                  252     406  62.1% -lh5- ccac Sep 11 00:27 fortnightly.rexx
---------- ----------- ------- ------- ------ ---------- ------------ ----------
 Total         2 files     658    1070  61.5%            Sep 10 21:54