The default stop date adjustment in a Service Defintion is <none> (see Date Adjustment to service stop (e.g., "as of") date in the Service stop adjustment). For elapsed time calculations, this is equivalent to not giving credit for the last day worked. For example, if you were hired on 1/1/2021 and needed to know how much service you had on 1/31/2021, you would not earn 1 month of service because you would be 1 day short: 31-1 = 30 (by default, you didn't "work" 1/31/2021). There are 31 days in January, but the default calculation gives you 31-1, or 30 days.

If you want to give credit for the last day worked, you need to create a Date Adjustment to add 1 day to the stop service date:

• Create a new Date Adjustment and call it "Plus 1 day". Click the Adjust date checkbox, and in the expression type: 

	#THIS + 1 ; add one day

• Save as a new Date Adjustment, and use it as the Date Adjustment to service stop (e.g., "as of") date in your Service Definition.

For example, using date subtraction for your elapsed time calculation, e.g., (y₂ - y₁) + (m₂ - m₁)/12 + (d₂ - d₁)/ 365.25), how much service did you earn in 2020, [1/1/2020,12/31/2020]? On 12/31/2020, using the date subtraction formula, you have (2020-2020) + (12-1)/12 + (31-1)/ 365.25 or 11/12 + 30/365.25, which is less than 1 year of service! By using a Plus 1 day stop date adjustment, the date subtraction becomes (2021-2020) + (1-1)/12 + (1-1)/365.25 = 1 + 0/12 + 0/365.25, or exactly 1 year of service.

Even when looking at how much service you earned in a month, e.g., [11/1/2020,11/30/2020], (2020-2020) + (11-11)/12 + (30-1)/365.25 or 29/365.25, which is less than 1 month. By using a Plus 1 day stop date adjustment, the date subtraction becomes [11/1/2020, 12/1/20201): (2020-2020) + (12-1)/12 + (1-1)/365.25 = 0 + 1/12 + 0/365.25, or  exactly 1 month of service.

Note: most people want the service on the end of each month to be 1/12, and not to vary by month. Without the Plus 1 day stop date adjustment, months with 31 days will have 30/365.25 service on the last day of the month, months with 30 days will have 29/365.25 service on the last day of the month, and February will have either 27/365.25 or 28/365.25 service on the the last day of the month. With the Plus 1 day stop date adjustment, on the end of each month, you will have exactly 1/12 service.

If you are using completed years or months for service, then this Plus 1 day service stop adjustment becomes even more important! You could handle this by rounding the service by measurement period, but there is always some weird situation where your service may not be correct. Why not get the underlying date subtraction to do exactly what you want (just use the Plus 1 day service stop adjustment)?