How to get the startTime attribute to line up layers on whole frames

I'm trying to use startTime to position layers in time. When trying to do this in comps at non-whole framerates such as 23.976 or 29.97 then the layers never quite align properly to the whole frame. for example with...

app.project.item(index).layer(index).startTime = 2;

the layer will sit between the 01:29s and 02:00s. Any ideas how to fix this?

Thanks!