The date that matters for your creditable service and pension calculation is your RSCD (Retirement Service Computation Date). This date matters because this is the date that OPM will use to calculate your retirement.
Unfortunately, there is no official document that shows your RSCD before you retire. Your Personal Statement of Benefits will have an estimate but that is just an estimate. You can use your SCD as an estimate as well but just know that it may be the same as your RSCD.
But again, OPM will calculate your RSCD from your SF-50’s. So you’d be able to calculate it by reviewing all your SF-50’s and finding the amount of time that you were official paying into the FERS retirement system.
Once you have your RSCD, calculate the amount of full years and months between that and your desired retirement date. Only full months count toward your creditable service so you’ll have to drop any extra days. 30 days is treated as a full month and OPM never rounds up.
For example, let’s say that the time between your RSCD and your planned retirement date is 22 years, 6 months, and 29 days. In this case, your creditable service would be 22 years and 6 months because the 29 extra days would drop off.