Solution : https://service.sap.com/sap/support/notes/91747 (SAP Service marketplace login required)
Key words : 
additional key words monthly contributionprogram, incorrect monthly contribution, billing plan date, billing plan type, monthly contribution, periodic billing, system determines, month = 30date, lv61au21function module, komp-anz_monate
Related Notes : 
       
| 92701 | Revenue recognition: Number of days per month | 
| 89429 | |
| 80069 | Pricing in billing plan: Monthly amount incorrect | 
| 62790 |