When keeping a diary, put a line directly under your last entry, leaving no gap. If you need to add to it, write another entry with time and date of when you're making it. If it's a recollection, note the time and date of the incident within the note itself, don't make it the entry date. This covers you and is transparent so that it doesn't look like you're inventing things. It will be invaluable for you if this goes to mediation. My diary got my old boss into a real pickle (he was too arrogant to keep notes on me and his interactions with me and only had one thing in writing - a nasty and vindictive email he sent me...yes, you read right, it was by email!) and even his lawyer (ha ha! I made him spend money on a lawyer for the afternoon!) was shaking his head at how I had snookered him with his client's own actions. Buy an exercise book and keep it on you at work (try not to leave it unattended - my favourite is one called a Fat Little Notebook).
I'm not jaded, I promise