Use the Date Arithmetic function The HP 100/200LX version of HP Calc comes with a Date Arithmetic application that lets you calculate the number of days between two dates. For example, lets say its October 17 and we want to know how many days till Christmas (Dec 25). From HP CALC press (MENU) Applications Date Calcs and follow these steps (this example assumes the date format is month/day/year):
Step 1: Key in the first date Enter it in the following format: MM.DDYYYY. For our example, enter July 4, 1995 as 07.041995. Press (F3) to store the date in the DATE1 field.
Step 2: Key in the second date. For our example, Dec. 25, 1995 is entered as 12.251995. Press (F4) to store the date in the DATE2 field.
Step 3: Calculate number of days. Press (F5) to calculate the number of days between the DATE1 and DATE2. The answer, DDAYS = 174, is displayed on the Calc Line at the bottom right of the display and next to Actual days between dates, the third line down from the top of the screen.
We can also use this application to calculate the number of days till our vacation, determine the day of the week for any date, and determine a future or prior date (i.e. 35 days before or after a specific date). For more on this function, look up Date Calculations in the index of your HP 100/200LX Users Guide.