Getting NaN when I try to find the Difference betweed @lastModifiedDate of Workflow and Current date

I have a requirement where I have to determine the difference in time when a particular workflow was last Run and current time (in minutes).

I query xtk:workflow schema in a JS activity in a workflow with "where" condition with the workflows Internal name and the fetch its @lastModifiedDate. However, when try to find the difference between the current date and lastModified date ( currentDate - lastModifiedDate) it gives NaN error. I tried converting the dates into milliseconds using parse() method. In this case current date gets converted to milliseconds but lastModifiedDate does not and returns NaN. I also tried converting lastModifiedDate into string, generate a date using the string [ var lastDate = new Date(lastModifiedDateString)] and then parse it  , but I get same NaN from lastModifiedDate but current date works fine. The string generated for lastModified is perfectly ok.

Please suggest how to get the time difference, in Javascript activity, between the last time the wkf had run and current time. (if possible in minutes)