If you're referring to billing from the 485 Form that the physician signs to order home health, I use the date the service started for either the initial or the recertification.
So, for initial certification "start of care date" (box 2) is the date you will use.
If it is a re-certification, use the "certification period" (box 3). For example, if the date in box 3 is "1/30/2014 to 3/30/2014", you would use 1/30/2014 (the first date the re-certification started).
Also, the way I find out if the patient is in an 60-day home health episode, I check Medicare's Endeavor system and look at the tab for home health.
Sometimes the HHA is lagging in putting the date in, so you might have to check back a few times...
G0179 for Re-certification
G0180 for Initial certification
This is the way I have billed for years, but I don't know where to find it in writing. The AAPC Coding Edge had a nice article by Kristine Cuddy. I don't know which month/year this came from. I tried to attach it, but couldn't, sorry.