Filter on date using variable

I have a variable for a date that is 60 days old called date60days.  I want to filter out any frequently used items from the launchpad gateway service where the last used date is greater than 60 days.  I am unsure how to utilize the variable in my filter.  I have tried the following:

$filter=myLastUsed gt datetime'" + date60days + "'&$orderby=myLastUsed desc",

I was able to do this in the javascript itself by using a condition on the for loop when adding the links as follows:

if (ostaticlinks[i].myLastUsed > date60days ) {

however, I'd rather filter this upfront rather than retrieving so much data from the backend.  Do I just not have the syntax correct for the $filter?