[0.25][3.1.190618-rc2] HTLC - Problem with "Extend HTLC"

**Describe the bug**
When I want to extend HTLC and I click on "Expiration date - Two days" or "Expiration date - One week", I see the date changing from the current one to two days ahead or a week. But after updating the time of the HTLC action, I see that the value I specified was added to the previous value, and not replaced by it.

- Expiration date was "One day"
- I want to change it to "Two days", I click on "Two days"
- But I get 3 days as a result.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to [HTLC](https://staging.bitshares.org/#/htlc)
2. Click on 'Create HTLC'
3. Fill out the forms
4. Click on 'Create'
5. Go to [HTLC](https://staging.bitshares.org/#/htlc)
6. Click on 'Extend'
7. Change the "Expiration date"
8. Click on 'Update'

**Expected behavior**
See changes in the value of "Expiration date" from "One day" to "Two days", and not the summation of these values. Or a more accurate description of this feature.

**Screenshots**

**Desktop:**
- Windows 10
- Browser: Firefox 67.0.4

**Additional context**
Not "Add seconds to claim period", but "Change seconds in claim period"
Or change the description in the form of "Extend HTLC"