Calculating the difference between two dates is a common requirement and one that you will likely need to do at some point. Below you will find the best practice for calculating the difference between two days, and the best practice for calculating the difference between two months.
While there are multiple ways to get to the answer you're looking for, often the simplest solution is the best. When calculating the difference between two dates (days or months), use this approach first:
For date formatted line items, use: line item 1 – line item 2.
For time formatted line items, use: START(line item 1) – START(line item 2).
Dates
1. Simple dates difference

2. Dates addition

3. Dates subtraction

Time periods
1. Time period difference

2. Months addition

3. Months subtraction

4. Years addition

5. Years subtraction

Keep 'superset' in mind when selecting your time period
If you choose a time period that is outside of the superset, it will display as blank. The superset is the earliest date from the model calendar and any time range through to the later of the last date of the calendar of any time range.
Example model:

Time:

Superset = Jan 17 - Dec 22:


If you go outside of that range:
You're now ready to calculate date differences in your daily practice. If you have additional questions or feedback, please add them to the comments below.