The Deja Vu is a Simple Partial seizure that commonly can precede a
full (right) Temporal Lobe or Complex Partial (newer term) Seizure.
Often the latter part can end in loss of consciousness when it ends (so
we're on the floor, if it generalized to a full CP).

  **If you can't tell about the Absences, those are usually included
in the Simple Partial seizure that can be the start of above CP or
Grand Mal type seizures, *or can happen on their own then pass...
(More detail on some of that can be found on the efa.org site or
Julie's Idaho Website, among some websites I posted in one post about a
week ago here.)

 To answer your question, you probably can't (tell), unless something
around your environment gives you some clues.   If you had planned to
watch a TV show at 10, and you wrote a note to remind you to change the
channel (the tv is on), and you look at the note, and it's 1015.. have
you just forgot about the note? in front of you, or did you have a 20+
minute absence?  (that's what mine were like)
 You hear water running, and when you go out to the kitchen, the Sink
has just started to overflow....  Did you just forget to stay with
that, or was that an absence?
 Here's a note you were making about a book you heard about on the
radio, and below it are about 30-100 scribbled lines that cover most of
the bottom 2/3 of the page---  who did That I wonder? .... the parts
you can read are in *your handwriting.
    Some or all of those are symptoms of Absences, or Simple Partial
szrs., but there could be others. (We have a tv advertisement running
here right now from the Alzheimer's Assoc. that suggests they can have
similar symptoms too--- forgetting or misplacing things).
    But it's for reasons like above that several of the Ep. "Safety
and Seizures" sites recommend using e.g. Electric Shavers,  Electric
Kettle with an Auto-Off feature, and other things that take some of the
risks out of Absences or Full seizures.

   How would you use above timer (in subject line) to count the
Duration if you're alone?  Did you mean something where you'd have to
Hold a button all day and if you Phased-out, would release it and start
a counter?   That seems like a lot of trouble, and many times our
autonomic systems might just **keep that button pressed for the *whole
duration of the absence, since it becomes a learned habit, and holding
the button down during an Absence is just as easy (or moreso)  as
holding it down while you're not...

   I only found out about Mine, before I was controlled, if I Had one
on the phone with someone, and apparently my voice and attention would
change, sometimes I'd 'dial the phone'  so they'd hear the tones since
we were already talking, then I'd hang the phone up!     About an hour
later, when I called one of them back, she was *really upset, as she
didn't know who to call or whether to call an ambulance and explain I
was in a locked house and what had happened..
   During those periods and until we're sure they're fully controlled,
they recommend if we swim we don't do it alone, and if they are
frequent we have someone to come along when we go shopping or go out
until we reach a medication? or some method of balance that reduces
those to near nil.  But unless we have someone who knows what our
absences look like, if your's were frequent, it's hard to know that you
are in fact fully controlled.    G./

Four years ago, I had a benign tumor removed from my right temporal
lobe.  Since then, I periodically have simple-partial seizures.  They
seem more or less like strong auras.
The first indication is a slight panic attack.  After that comes the
deja vu and the strange smells.  The deja vu seems so real... I swear I
can say the words coming out of your mouth as you say them.  The smells
often remind me of the past.  The whole thing last 30 to 60 seconds.
Sometimes, I actually wish it would last a bit longer so I could figure
out what the smell is.  Sometimes it's similar to play dough or burning
rubber... but I can never quite put my finger on it.  Anyway, I have
the overwhelming urge to be alone when these happen. However, I doubt
you'd know I was having one unless I told you.  Strange stuff.  Good
luck.

John