The introduction of the Act/Act day count convention. This would mean that the actual number of days in the period divided by the number of days in the current year are used. For example:
01.01.19 to 01.02.19 = 31/365
01.02.19 to 01.03.19 = 28/365
01.01.20 to 01.02.20 = 31/366
01.02.20 to 01.03.20 = 29/366
This is also mentioned briefly here: http://www.deltaquants.com/day-count-conventions
Some organisations require interest calculations which take the real number of days in both the given period and year into account.