Texas Instruments TI-83 Manual Book page 414

Days between
Dates
A-58 Tables and Reference Information
With the function, you can enter or compute a date
dbd(
within the range Jan. 1, 1950, through Dec. 31, 2049.
Actual/actual day-count method (assumes actual
number of days per month and actual number of days per
year):
dbd( (days between dates) =
Number of Days II - Number of Days I
Number of Days I = (Y1 - YB)
Number of Days II = (Y2 - YB)
where: M1 = month of first date
DT1 = day of first date
Y1 = year of first date
M2 = month of second date
DT2 = day of second date
Y2 = year of second date
MB = base month (January)
DB = base day (1)
YB = base year (first year after leap year)
365
+ (number of days MB to M1)
+ DT1
( Y 1 YB )
+
4
365
+ (number of days MB to M2)
+ DT2
( Y 2 YB )
+
4