questo é quello che ho creato:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>Untitled</title>
</head>

<body>
inserisci la tua data di nascita
<FORM NAME="eta">
<SELECT name=giorno size=1>
<OPTION selected value=0>
<OPTION value=1> 1
<OPTION value=2> 2
<OPTION value=3> 3
<OPTION value=4> 4
<OPTION value=5> 5
<OPTION value=6> 6
<OPTION value=7> 7
<OPTION value=8> 8
<OPTION value=9> 9
<OPTION value=10> 10
<OPTION value=11> 11
<OPTION value=12> 12
<OPTION value=13> 13
<OPTION value=14> 14
<OPTION value=15> 15
<OPTION value=16> 16
<OPTION value=17> 17
<OPTION value=18> 18
<OPTION value=19> 19
<OPTION value=20> 20
<OPTION value=21> 21
<OPTION value=22> 22
<OPTION value=23> 23
<OPTION value=24> 24
<OPTION value=25> 25
<OPTION value=26> 26
<OPTION value=27> 27
<OPTION value=28> 28
<OPTION value=29> 29
<OPTION value=30> 30
<OPTION value=31> 31
</OPTION>
</SELECT>
<INPUT name=correct size=31 type=hidden >


<SELECT name=mese size=1>
<OPTION selected value=0>
<OPTION value=1> Gennaio
<OPTION value=2> Febbraio
<OPTION value=3> Marso
<OPTION value=4> Aprile
<OPTION value=5> Maggio
<OPTION value=6> Giugno
<OPTION value=7> Luglio
<OPTION value=8> Agosto
<OPTION value=9> Settembre
<OPTION value=10> Ottobre
<OPTION value=11> Novembre
<OPTION value=12> Dicembre
</OPTION>
</SELECT>
<INPUT name=correct size=12 type=hidden >

<SELECT name=anno size=1>
<OPTION selected value=0>
<OPTION value=1> 1900
<OPTION value=2> 1901
<OPTION value=3> 1902
<OPTION value=4> 1903
<OPTION value=5> 1904
<OPTION value=6> 1905
<OPTION value=7> 1906
<OPTION value=8> 1907
<OPTION value=9> 1908
<OPTION value=10> 1909
<OPTION value=11> 1910
<OPTION value=12> 1911
<OPTION value=13> 1912
<OPTION value=14> 1913
<OPTION value=15> 1914
<OPTION value=16> 1915
<OPTION value=17> 1916
<OPTION value=18> 1917
<OPTION value=19> 1918
<OPTION value=20> 1919
<OPTION value=21> 1920
<OPTION value=22> 1921
<OPTION value=23> 1922
<OPTION value=24> 1923
<OPTION value=25> 1924
<OPTION value=26> 1925
<OPTION value=27> 1926
<OPTION value=28> 1927
<OPTION value=29> 1928
<OPTION value=30> 1929
<OPTION value=31> 1930
<OPTION value=32> 1931
<OPTION value=33> 1932
<OPTION value=34> 1933
<OPTION value=35> 1934
<OPTION value=36> 1935
<OPTION value=37> 1936
<OPTION value=38> 1937
<OPTION value=39> 1938
<OPTION value=40> 1939
<OPTION value=41> 1940
<OPTION value=42> 1941
<OPTION value=43> 1942
<OPTION value=44> 1943
<OPTION value=45> 1944
<OPTION value=46> 1945
<OPTION value=47> 1946
<OPTION value=48> 1947
<OPTION value=49> 1948
<OPTION value=50> 1949
<OPTION value=51> 1950
<OPTION value=52> 1951
<OPTION value=53> 1952
<OPTION value=54> 1953
<OPTION value=55> 1954
<OPTION value=56> 1955
<OPTION value=57> 1956
<OPTION value=58> 1957
<OPTION value=59> 1958
<OPTION value=60> 1959
<OPTION value=61> 1960
<OPTION value=62> 1961
<OPTION value=63> 1962
<OPTION value=64> 1963
<OPTION value=65> 1964
<OPTION value=66> 1965
<OPTION value=67> 1966
<OPTION value=68> 1967
<OPTION value=69> 1968
<OPTION value=70> 1969
<OPTION value=71> 1970
<OPTION value=72> 1971
<OPTION value=73> 1972
<OPTION value=74> 1973
<OPTION value=75> 1974
<OPTION value=76> 1975
<OPTION value=77> 1976
<OPTION value=78> 1977
<OPTION value=79> 1978
<OPTION value=80> 1979
<OPTION value=81> 1980
<OPTION value=82> 1981
<OPTION value=83> 1982
<OPTION value=84> 1983
<OPTION value=85> 1984
<OPTION value=86> 1985
<OPTION value=87> 1986
<OPTION value=88> 1987
<OPTION value=89> 1988
<OPTION value=90> 1989
<OPTION value=91> 1990
<OPTION value=92> 1991
<OPTION value=93> 1992
<OPTION value=94> 1993
<OPTION value=95> 1994
<OPTION value=96> 1995
<OPTION value=97> 1996
<OPTION value=98> 1997
<OPTION value=99> 1998
<OPTION value=100> 1999
<OPTION value=102> 2000
<OPTION value=103> 2001
<OPTION value=104> 2002
<OPTION value=105> 2003
<OPTION value=106> 2004
<OPTION value=107> 2005
<OPTION value=108> 2006
<OPTION value=109> 2007
<OPTION value=110> 2008
</OPTION>
</SELECT>
<INPUT name=correct size=110 type=hidden >
</FORM>


<script>
//estraggo i valori dai form
var giornof = document.eta.giorno.value
var mesef = document.eta.mese.value
var annof = document.eta.anno.value
//coverto annof
var anno = annof + 1899
//ottengo la data attuale
oggi = new Date
var giornoa = oggi.getDate();
var mesea = oggi.getMonth();
var annoa = oggi.getFullYear();
mesea++
document.write (anno);

</script>
</body>
</html>
il mio problema e che non riesco a sommare questo:
var anno = annof + 1899
poiche mi da come risultato un concatenamento e non una somma algebrica...

per favore mi potete aiutare?