Search function and lastmodifieddate

Hi All
I'm tying to do a search on existing tickets where lastmodifieddate is within 24 hours:


var nowDate = new Date();
var nowTimestamp = nowDate.getTime();
var  BST_Timestamp = nowTimestamp + (1000 * 60 * 60 * 8);
var  twentyfour_ago_timestamp = BST_Timestamp - (1000 * 60 * 60 * 24);
var  twentyfour_ago_date=new Date(twentyfour_ago_timestamp);
   
var filters=[];
filters[0]=new nlobjSearchFilter("company",null,"is",customerName);
filters[1]=new nlobjSearchFilter("custevent_bb1_circuitname",null,"is",reportDetails);
filters[2]=new nlobjSearchFilter("lastmodifieddate",null,"greaterthan",twentyfour_ago_date);
    
When I just do filter [0] and [1], it works fine, but adding [2] gives me "UNEXPECTED_ERROR" in the script debug log.
Any ideas? From what I can find, it may be that lastmodifieddate is in a different datetime format to twentyfour_ago_date ???