You have written code to bind the selected period. When you change through the calendar date in one field, the second date is automatically recalculated according to the selected period.
When you put in the first field, for example date APR 27, 2018 the end date will be calculated by your code
this.dateEnd = moment(value).add(1, this.selectedPeriod).toDate()
for today and will add 1 day to start date, this will be the value of the final 28 APR 2018
the week will be added 1 week - ending date may 4, 2018
per month - 1 month - final date 27 may 2018
In this case, it is not very clear what the problem is