There are two time-weighted approximation formulas that you’re probably at least somewhat familiar with: Modified Dietz and Modified BAI. Neither of them began life as being labeled “Modified.” I believe it was probably the Association for Investment Management & Research (AIMR), now the CFA Institute, who decided to use these terms when the AIMR-Performance Presentation Standards (AIMR-PPS®) were launched 30 years ago (AIMR (1993)). This article will touch upon, clarify and contrast these formulas.