Boolean Problem

I've written some code that creates an sql statement in a string to update a boolean value in an access database, however, I dont know what value to give the field to update the boolean value in the access database.
I'll paste some of the code so it makes more sense.....
(note: the 'Booked' Field is the Boolean)

strSql = strSql & " UPDATE HolliesBookTbl SET Booked = '-1' WHERE "

'loop through the checkboxes which were ticked
for i = 0 to ubound(arrVals)
if i = 0 then
strSql = strSql & " DateID = '"& arrVals(i) &"'"
else
'only add the " AND " if this is not the first value
strSql = strSql & " AND DateID = '"& arrVals(i) &"'"
end if

I've been pullinhg my hair out over this