Interest under sections 215 and 217 cannot be increased on assessment variation; compute interest from April first to regular assessment. Interest under sections 215 and 217 cannot be increased due to variation in assessed income on appeal, revision or rectification; reduction is permitted under section 215(3). Interest is to be computed from the first day of April following the previous year up to the date of regular assessment, where regular assessment means assessments made under the provisions corresponding to sections 143 or 144.