In the same way as some nappies have paper strips that change colour
when they get wet, or toothbrushes whose colour goes pale when they
have been used for a while, or ph paper that changes according to
acidity; is there a similarly disposable product that changes colour
only after a specific time, between (say) 3 days and 30 days?  It
needs to be able to indicate when a prescribed time has elapsed within
a tolerance of a day or 2.  I'm not interested in clocks etc; it must
be simple and cheap enough to be disposable.  I would like to know who
makes it and what applications use it.

Hi pinky6666-ga, and thanks for your question.

Probably the product closest to your description is Timestrip:
http://www.timestrip.com/home.html

This is a disposable device that indicates a range of times from under
1 day up to 6 months.  The primary use at this time is in the food
industry (to indicate how long something has been on the shelf or open
in the refrigerator), but there are several options that work at room
temperature.

This device uses capillary action to allow a colored liquid to diffuse
through a carrier at a constant rate, giving constant time
information.  To start it, you peel off the backing paper and squeeze
the bubble pack.  These originate in the UK, and you can purchase them
in the US from DayMark:

http://store.daymark.biz/searchadv.aspx?IsSubmit=true&SearchTerm=timestrip&submit.x=0&submit.y=0

A 25-pack of 3-day Cooler Timestrips costs $4.99.

The FridgeStrips and FreezerStrips have markings for days or months:
http://www.timestrip.com/OPcatalogue.php


Here is an article about this product from PackWorld.com:
http://www.packworld.com/cds_search.html?rec_id=18792&eclip=yes


===========
Search terms:
paper time indicator
timestrip
===========

I hope this information is useful.  Please feel free to request any
clarification prior to rating.

     -welte-ga

Precisely what I was looking for.

What is the application?  Would a time stamp serve the same purpose? 
There are things that change color, but they must be exposed to
something like UV light or heat or some other physical force or energy
for a period of time.

What you are probably looking for is a material that changes color on
exposure to light or air.

Ordinary food coloring is sensitive to light, and fades over time, so
it may be a cheap thing you can experiment with.  I don't know how
long it takes, but I'm sure by using different concentrations,
different colors, and the like, you can tune it to get different fade
times.

A couple of years or so ago Flexplay introduced its disposable DVD
format. Basically, the surface of a disc is coated with a chemical
which reacts with ambient oxygen in the atmosphere, and after a
certain period of time the resultant compound is opaque. At sea level
concentration of O2 this occurs at approximately 48 hours.

   But the time period can be adjusted by altering the concentration
of the reactive chemical on the disc. Probably it could be calibrated
to turn opaque very reliably at 30 days. Of course, for a cheap alarm
device such as you describe it would be deposited on an inert piece of
plastic. You could consider contacting Flexplay
(http://www.flexplay.com/index.htm) and inquiring as to the
adaptibility of their technology for your purposes.

For the product / final application to your requirement a few things
must be assured, sort of specs

1. There shall be an activation trigger to start countdown, e.g.
wetting of pH indicating paper
2. Interaction with and only with intended environment e.g. tooth
brush bristles being "used" for brushing and not affected by lying
idle.
3. Adequate shelf-life before use.
4. Reliable indication of event completion.

However in your question since you have not specified the interaction
medium or agent, it is assumed that you wish to count only time after
initial activation event.

Well, I will suggest you try some plucked leaves or flowers. They
change colour in absence of their native environment. But be careful
about the prevalent season and so many other factors. I think you may
be able to evolve a working model to suit your requirements.
Working principle
1. Activation event is plucking from tree or removal from preservation
environment (thaw / de-freeze or take out from a jar of formaldehyde
or ....may be so many techniques)
2. Interaction with intended environment = absence from native environment.
3. Shelf life is model based and resource deployment dependent.
4. Reliability, well this is quality assurance and can be mastered.

I have attempted to evolve answer to your problem by logical thinking,
and I am confident it can be made to work but most important part is
that finally it should work for YOU an your intended application.

Please feel free to seek any clarification if need be!