GG is right it is the decline in cognitive abilities, an inability to put two and two together, that indicates the onset of dementia. I often forget what day it is but a quick check of the calendar or computer puts me right, whereas DH has lost the ability to 'read' the calendar, operate the remote, etc. etc.
His consultant explained to us why the ability to 'make a cup of tea' is often used as a diagnostic test. There are apparently 40 steps involved in what seems such a simple task - starting with deciding to have tea as opposed to coffee, locating the cup, tea bags, milk, sugar, filling the kettle, switching it on, remembering your own 'recipe' and so on...... DH has recently lost the ability to do this successfully, (even at home where everything needed is kept together in the same corner) after some months of a hit and miss approach when he would bung a tea bag into my coffee, to 'be on the safe side'.
If you keep forgetting things e.g. names and addresses for the Christmas Cards but can use the phone book/an old list etc. to fill in the gaps then this is not dementia. If you cannot open a card without first locating your special letter opener, which is on the window sill beside you, or simply tearing it open then this is dementia. Bearing in mind that there are numerous types, which present differently and other conditions which cause confusion and memory loss too.