Interval must be one of the following: "m": month; "d": days; "y": complete years; "ym": month excluding years; "yd": days excluding years; "md": days excluding months and years
De tuskenskoft moat ien fan de folgjende wêzen: "m": moannen; "d": dagen; "y": folsleine jieren; "ym": moannen sûnder jieren; "yd": dagen sûnder jieren; "md": dagen sûnder moannen en jieren