is it possible to infer timezone from us zipcodes using a simple
algorithm? If so, what is that algorithm?

I don't think so. However I think this information is available in
Micorsoft Mapoint. I believe on the Product website you can contact
the Product Manager, check but I am almost positive that you can get
the time zone for every zipcode.

Yes, the simplest solution is to resolve a ZIP code into a county. US
Standard Time boundaries are established by the Department Of
Transportation, and they fall along state, coastal, and county lines.
Most States belong to a single time zone so resolving a ZIP code by
state would most often be sufficient to determine a time zone.
However, some states cross time zone boundaries, but they are divided
by counties. Probably the best way to handle this would be to maintain
a databse of exception counties to determine if a county belongs to a
minority. For example, there are a small number of counties that fall
in the Central Time Zone versus those that fall in the Eastern Time
Zone. Hence, consider any county in Florida to belong to the Eastern
Time Zone except those that appear in your database. United States
Postal Services offer Web tools Address Information APIs to help
developers resolve a ZIP Code into a city and state. There may be an
API to help you determine a county. For an authoritative source on
time zone boundaries consult the Code of Federal Regulations
49CFR71.1-14. You can find this at
http://www.gpoaccess.gov/cfr/retrieve.html.

For additional information:
http://geography.about.com/library/misc/ntimezones.htm
http://nationalatlas.gov/mld/timeznp.html
http://www.usps.com

There is also some interesting information from the Census Bureau.
There is something called Zip Code Tabulation Areas (ZCTA) used to
divide land into regions. This uses the first three digits of US zip
codes. This would most likely be the best way to map a zip code to a
county, but it would require a fairly large database. For more
information see http://www.census.gov/geo/ZCTA/zcta.html.

These might be helpful:

For any valid ZIP code, find the city and state, plus:
http://www.zipinfo.com/search/zipcode.htm
-county name and FIPS code 
-area code 
-time zone  
-latitude and longitude 
-MSA/PMSA 


And here's a database:

ZATDB - Zip Code/Area Code/Time Zone Database  
http://www.kjlsoftware.com/zatdb.html