It depends
How old are the charts? What exactly is missing?
If it's a note that was handwritten or dictated by the physician him/herself, but s/he hadn't signed it, then I see no problem with signing it now.
If a date of service is missing or incorrect, but you can clearly identify what the correct date is from other items in the patient chart or office records, then I would amend the note to include the correct date of service. The amendment should be dated & signed with the date the correction is made. For example:
Patient name, MR #
AMENDMENT to office visit note of 07-04-09
The correct date of service is 07-14-09, verified through office schedule and patient sign-in sheet of that date. Our office was closed on 07-04-09
Signed: Doctor
Dated: 09-03-09 (i.e. the date the correction is made)
If this is a situation where a resident or NP recorded much of the documentation, but the physician portion is missing ... I wouldn't go "back" more than a week or two. Frankly I wouldn't trust that the physician's memory of what actually occured would be sufficiently accurate to properly document his/her participation. I would, instead, correct the claim ... might be unbillable or might be able to be billed under the NP or PA's own name, number.
These kinds of issues are fairly common. And they are a very good reason for continual monitoring/auditing to help educate the providers on documentation requirements and help them develop good documentation habits.
(I live in a fantasy world, don't you know ... LOL)
F Tessa Bartels, CPC, CEMC