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I currently have an empty list that contains 2 fields of the type text (of which one is a rename of the title column), one field which has a data-time and another field which has a field of type date/time.

Now I have a document library into which the l have added custom lookup columns. My problem right now is I am unable to add lookups for non-text columns. Is this a limitation of WSS 3.0 oris there a work around? 

There are definitely limits on what kinds of data types can be used as lookup column data sources.

The workaround is to use one of the support types (not so useful in your case) or create a custom data type that supports this, which is a lot of coding.

Someone may have better ideas.

--Paul Galvin, Conchango
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Thanks for confirming my suspicions.

Is it possible to modify the native properties itself? I am working on something which requires the list to be looked up & I am disappointed that a simple date time field cannot be looked up. I am unclear on the limitation - is it a definition issue, since you mention custom type - or is it an issue with the lookup mechanism itself?

Right now it's sounding like a definition issue in which case I am really tempted to modify the master type itself, that is if I can locate the element. I am looking at the definitions of date/time and other aspects @ "%program files%\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\FEATURES\fields\fieldswss.xml" 

Text is defined as:

    <Field ID="{c042a256-787d-4a6f-8a8a-cf6ab767f12d}"
        Name="ContentType"
        SourceID="http://schemas.microsoft.com/sharepoint/v3"
        StaticName="ContentType"
        Group="_Hidden"
        RowOrdinal="0"
        Type="Text"
        DisplayName="$Resources:core,Content_Type;"
        ReadOnly="TRUE"
        Sealed="TRUE"
        ColName="tp_ContentType"
        PITarget="MicrosoftWindowsSharePointServices"
        PIAttribute="ContentTypeID">
    </Field>

Now some where in this file is the definition for date time but I'm not seeing it. I need to get a good xml editor other than notepad. I am also considering the use of calculated columns but I guess I will be getting a pure text result.

EDIT:

Found this article and found out that it's only text that you can lookup according to that. Now he does mention the DVWP but I have not worked with the DVWP.