Possible Duplicate:
Add “Retrieved”, “Last accessed” or similar information to authoryear in biblatex
I use JabRef 2.8b2 to organize my bibliography references. I used the latex makebst command to create a custom bibliography style (uploaded to pastebin.com). The custom style produces the following output.

However, I would like to add the access date to the URL like masson asked before. I imagine that the style could read the date from the timestamp field of the corresponding *.bib file. Here is the plain section from the *.bib file for the aformentioned example output.
@MISC{obama-opengov,
author = {Barack Obama},
title = {Transparency and Open Government},
howpublished = {Website},
month = {January},
year = {2009},
note = {Verified via archive.org},
timestamp = {2012.03.28},
url = {http://www.whitehouse.gov/the_press_office/TransparencyandOpenGovernment/}
}
Question:
How can I add something like Last accessed at 12-04-04 to the style? Did I miss a question in makebst to customize it?
I am not insisting on my bib style if you can recommend another with a similar output. The optimal output would look something like this:
[5] Barack Obama: Transparency and Open Government. Website (January 2009).
URL: http://www.whitehouse.gov/the_press_office/TransparencyandOpenGovernment/.
Verified via archive.org. Last accessed: 2012-04-04.
Further unsucessful attempts:
I tried to use the mla-good.bst and mla-goodURL.bst linked in massons' question. I also tried to manually modify them as suggested. However, the date does not print although I added values to the urldate field via JabRef. I also tried to replace the fieldname with timestamp which did not work either.
Second, I tried natdin.bst by Stefan Macke who uses the lastchecked variable for the same purpose. Same result.
After all, I cannot print any date. Further, I cannot see a lastchecked field in JabRef. My version of JabRef writes the current date at which I add the entry to the database automatically to field timestamp. Therefore, I would prefer to use this field.