Calculate approximate birthdate based on current age

I have an Access 2010 database used to track information on animals that come into our shelter.  I need a way to keep better track of their age.  Currently we enter the age based on what age it was when it first came into contact with us.  Because we very rarely ever know the birthdate, the age is approximate based on our best guess.  The problem is that that age never changes no matter how long they've been here.  This is especially problematic for our animals less than a year old.  We have ages ranging from hours, days, weeks, months, to years.  I created a table a long time ago that converts the age they select from the drop down into years.  So when they choose 3 months it converts it into 0.25.  I'm using the field with the 0.25 data to figure out the birthdate using [BirthDate] = DateAdd("yyyy", -[Age], Date) but it only works if the animals is older than 1, otherwise it sees the age as zero and just enters today's date for the birthdate.  What do I need to change in order to make this work?

Thank you
Who is Participating?

CIOCommented:
First, you should record the birthdate and not the age.

Then, let the user choose the unit of age: Hours, Days, Weeks, Months, Years.
If you use a combobox, let it have the columns:

"h";"Hours"
"d";"Days"
"ww";"Weeks"
"m";"Months"
"yyyy";"Years"

You will hide the first column and display the second.
The first column is used as the date interval to calculate the birthdate.

Now, calculate the birthdate from the selected interval and the value:

/gustav
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Commented:
why not add a birthdate field and insert approx. date if true date is unknown?
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Author Commented:
Because a lot of the time the person bringing them in doesn't know an approximate birthdate and will just say it's 8 years old.  I don't want the staff to have to then figure out what 8 years ago was.  Trust me, all sorts of wrong birthdates will be entered.
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Commented:
if you leverage the date you first added the animal, you can derive the years.
do you store first-appointment date?
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Author Commented:
Yes.
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Commented:
then age = DateDiff("yyyy", #3/4/2010#, Now())
substitute  #3/4/2010#, for your startdate field

don't store age. this is derived and displayed on the form/report.
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Author Commented:
If I had a start date field that related to its age i could do this but the start date field is the date of our first interaction with the animal.... it could already be many years old by then.
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Commented:
if you enter an age field you can add this value to the difference in years between start-date and now

displayage = age +  DateDiff("yyyy", #3/4/2010#, Now())
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Author Commented:
That still doesn't work when the age is less than 1
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Commented:
displayage = IIF(NZ(age,0)=0,1,age) +  DateDiff("yyyy", #3/4/2010#, Now())
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Commented:
animallover indicated they don't always know the date - hence the need to store approx. age.
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CIOCommented:
No. They have the age, then will calculate and store the DOB.
Then the actual age can be calculated at any future date.

/gustav
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Commented:
'Currently we enter the age based on what age it was when it first came into contact with us.  Because we very rarely ever know the birthdate, the age is approximate based on our best guess'
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CIOCommented:
Yes. And then:

> The problem is that that age never changes no matter how long they've been here.
> This is especially problematic for our animals less than a year old.

Thus, calculate the (approximate) DOB, store that, and then the actual age can be retrieved at any time later.

/gustav
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Commented:
displayage = IIF(NZ(age,0)=0,1,age) +  DateDiff("yyyy", #3/4/2010#, Now())  will derive/display the current 'age', based on the stored 'age' (years)

better still (based on original spec):
[BirthDate] = IIF([Age] > 0, DateAdd("yyyy", -[Age], Date), 1)
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CIOCommented:
Except for the syntax error, that would require the age to be stored relative to 2010-04-03.
Further, you won't have the age with a resolution less than one year.
Finally, the age would only change with the calendar year.

My method is much simpler and won't fail.

/gustav
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Author Commented:
Yes... we don't really need an accurate birthdate for animals over a year old but for those less than 6 months there are a lot of determinations made for that animal where age is a big factor.  We need to both be able to figure out the birthdate from the age at first interaction and then be able to update the age based on that birthdate.  Age is much more important to the end user and needs to be readily seen and available without having to go to a calendar to figure it out.  I was already using a combo box for the age but it wasn't broken down into columns as you suggested.  I am going to change it to your suggestion (Gustav) and give it a go.  Thank you for your solution!
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CIOCommented:
You are welcome!

/gustav
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