An ADD-like condition helped me screw up at work big time. How can I protect myself?

An ADD-like endocrine disorder isn't a mental health issue, it's an endocrine disorder. You're not dragging mental health into it, you're explaining to your boss that you've fixed your underlying health condition and are ready to return to your normal excellent performance levels, backed with documentation from your doctor and, if needed, an endocrine specialist.

Also not answering the question: If your email is through the agency you consult for, the agency may be required to keep the email for open records purposes. In other words, there may be a backup somewhere despite your inbox cleaning issues. Sort of answering the question: At a previous job, I screwed up my expense reports, and procedure required that company security investigate me. My job was on the line. However, I just clearly explained to the investigators how I screwed up and offered them an action plan to make them comfortable that I wouldn't screw up again. I didn't produce actual receipts, or anything like that (part of the screwup) but did logically explain how I got myself in such a screwed up position. I took full responsibility for my mistakes. I got a smaller bonus that year than I otherwise would have, but I kept my job. Because I addressed the problem head on, I also was able to fix my personal deficiency to my own benefit and make myself a better person (and employee). If you continue to be an excellent employee, confessing and fixing the issue should only be a short term hit on your reputation. In this case, you can explain how you already fixed the problem on your own initiative, putting you in a better place than I was.

I would avoid using the term ADD at all, and talk about it only as effects of hypothyroidism--people still are more likely to have a dismissive response to ADD ("It's not a real disease" bla bla) than they are for hypothyroidism, which has effects that can show up in chemical tests. You can explain the symptoms and how they affected you and how you worked with your doctor and they gave you medication that has helped, and look how it has helped: example! Think of it this way: hypothyroidism isn't a mental condition, it just has side effects which affect your mental capabilities. Having a cold will cause the exact same symptoms you mentioned, but nobody is going to claim that having a cold makes you mentally ill! I am also personally distressed by the thought that they may be perfectly willing to fire an excellent worker because of some timesheet questions. But then again, I hate the fact that most jobs reward you more for the time you spend sitting at your desk than the amount of work you do while you're sitting there. Also, look into saving copies of your email locally. If you use Outlook, look into .pst files, if you use something else . . . there should be options there too.

There are a number of logs of your time. Do you work in a secured area? Your badge should identify when you entered and exited the building. There will be a few times when you entered with others, but the general trend will be there. Also, your computer login and logouts can likely be pulled. Do you access or upload files to servers? Those files will have timestamps. You won't get a definitive picture but you should be able to show most of your time. About your history of timekeeping problems, I'd suggest you disclose that you had a medical issue and can provide doctor's records of diagnosis and treatment. Otherwise, you just look like you we're being a flake. I'd wonder when your attention is going to wander again. A medical condition isn't a source of shame.

I can't address many of your issues but I will take a stab at the documentation: Do you have saved files? Documents that were created or modified at a certain time? Activity logs on any shared environments (wikis? electronic libraries, etc). any colleagues who might have the emails you sent them? Any archives of emails? Good Luck.

I agree with what others said about checking your event viewer on your local PC (assuming you login immediately in the morning). A logon event is generated every time and since workstations aren't typically audited to the extreme of servers, it is common to have events go back years. However, I believe you need to be an administrator to view the Security log. Domain Controller event logs will overwrite themselves very quickly unless they're archived or exported to a 3rd party product so the server event log may not help much. As for remembering to log into the time clock system every day, you could try a trick I used when my last company introduced an online time clock. Create a shortcut for the time clock web site and then save that shortcut to your Startup folder on your workstation. Every time you login, the site will automatically launch. IANAD, but I remember reading an article about mental health issues at work (and I agree with others that this is clearly not mental health). The article said it may be beneficial to disclose your condition to your boss or HR but it must be done before it affects your work. You certainly have less credibility now that you're already in trouble.

Do you turn your computer on when you get in? If so, "Event Viewer", in "Administrative Tools" will save your life. It's the only way I ever manage to do my timesheet, and it goes back a year on my computer.

I don't believe you have a chance in hell of defending yourself if you don't have any supporting documentation for your billable hours. When I was a consultant (not even in the government) I kept a daily (hourly) log of my activities (Yay http://www.ataglance.com/webapp/wcs/stores/servlet/category3_10052_10002_46717_-1_Y_46703_10052_Y!). I realize this won't help your current situation, but may be of help to you in future endeavors and those reading this for similar assistance. Unfortunately, I see this as a "you've made your bed, now it's time to sleep in it" type situation. You live, you learn, you move on and hope to not make the same "serious mistakes" again. Good luck!

I agree with those who suggest that this is not a mental health disorder, and that you do yourself a disservice by presenting it as anything akin to one. If you decide to talk about it I would talk about it in terms of symptoms and solutions. I am also personally distressed by the thought that they may be perfectly willing to fire an excellent worker because of some timesheet questions. The OP is correct to take this seriously. Your employer is likely thinking of this as a potential case of fraud, and rightly so. It sounds like they're paying you by the hour and not the deliverable. Everyone has a right to know if your workload is too low, too high, or just right.