A simplistic answer is:

// Get a date object for the current time

var d = new Date();



// Set it to one month ago

d.setMonth(d.getMonth() - 1);



// Zero the time component

d.setHours(0, 0, 0, 0);



// Get the time value in milliseconds and convert to seconds

console.log(d/1000|0);

Note that if you subtract one month from 31 July you get 31 June, which will be converted to 1 July. similarly, 31 March will go to 31 February which will convert to 2 or 3 March depending on whether it's in a leap year or not.

So you need to check the month:

var d = new Date();

var m = d.getMonth();

d.setMonth(d.getMonth() - 1);



// If still in same month, set date to last day of 

// previous month

if (d.getMonth() == m) d.setDate(0);

d.setHours(0, 0, 0, 0);



// Get the time value in milliseconds and convert to seconds

console.log(d / 1000 | 0);

Note that JavaScript time values are in milliseconds since 1970-01-01T00:00:00Z, whereas UNIX time values are in seconds since the same epoch, hence the division by 1000.