Regarding a 20-year retirement, is there a combination of Reserve and AD time in service, or does it just go off active duty time?

RESERVE: Assuming good reserve years, you are probably 12 years away from a 20 year letter (with full, paid retirement around age 60, give or take some reserve mobilization time). The AC years count as "good years", of which you need 20. They also really bump up your retirement points, which will help determine you pay, once you get the 20 year mark.

ACTIVE: You are probably 14-15.75 years away from a 20 year regular retirement, depending on how your current service is sequenced. Take all of your earned reserve retirement points and divide by 360 (no, not 365/6). That is your number of equivalent days of AD service. Add it to your AC time to get your effective active time.

@SGT brooks porter SSG (Join to see) it depends on the status that you were in during your Reserve service. If you were activated (on full time orders) the entire time, the full four years count towards an ‘active’ retirement. If not, and you have four ‘good years’ (attended all required drills), the time would count towards a ‘Reserve’ retirement that can be claimed at age 60. There are also other factors to consider, like ‘Reduced Age Retirement’ where certain types of orders (ADOS, ADSW, MPA, RPA, etc.) may count towards a reduced age ‘Reserve Retirement’ as low as age 50. There is also the new active retirement system as a point to consider, recommend that you glean as much information from RP as you can and discuss with your mentor(s) about a plan. If you don’t currently have a Reserve mentor, I challenge you to seek out at least one worthy mentor this year.

The shorts answer is that all your time counts and rolls together. So you should have 8 years credit towards a Reserve retirement. Unless of course if you have any “bad” years while you were a Reserve. A bad year would mean you incurred less than 50 points for that year.