How do you determine the date of a box 85 medical expense?

When claiming medical expenses as listed in box 85 of a T4, how can you determine the date of the expense(s)? It would be helpful to know the date of the box 85 medical expense to determine the 12-month period for your claim and as well, in case you decide to only claim a portion of the box 85 medical expense.

Do most people claim the full amount from box 85? i.e. is it not necessary to only claim a portion of box 85?