Calculating period field error

Hi, i have strange problem with Period type field.

I've created such a workflow to track time of issue:

action: function(ctx) {
var issue = ctx.issue;
var field = issue.fields;
var est = field["Планируемая дата устранения"] - field["Дата выявления"];
field.Estimation = timeDate.toPeriod(est);
});

When i check value of est variable - it's correct in ms = 144000000, and correct in period = PT40H, but when i put  this var in field i want, it changes to 1w.