Date is not coming in proper format in partition chart

While using partition chart with BigQuery as data source the date which is of format “2020-05-27” i.e yyyy-mm-dd in superset gets displayed as epoch time. While running the same query on sqllab,bigquery or csv export from superset give it in proper format (yyyy-mm-dd)

Expected results

expected “date” to be displayed in “yyyy-mm-dd”

Actual results

date is getting displayed in epoch format

Screenshots

How to reproduce the bug

1. Creating a date column in format “yyyy-mm-dd”
2. 
3. Sqllab and csv export from superset gives the date in proper format i.e “yyyy-mm-dd”

Environment

(please complete the following information):

• superset version: superset version — 0.35.1
• python version: python --version ---- 3.6.9
• node.js version: node -v
• npm version: npm -v

Checklist

Make sure these boxes are checked before submitting your issue - thank you!

• I have checked the superset logs for python stacktraces and included it here as text if there are any.
• I have reproduced the issue with at least the latest released version of superset.
• I have checked the issue tracker for the same issue and I haven’t found one similar.

Additional context

Add any other context about the problem here.

[willbarrett]

We have confirmed that this is a bug on the current master branch. As this is an open source project, we do not have SLAs for fixing bugs. An existing contributor may contribute a patch for this issue, or we are happy to review pull requests from the broader community. If you’d like to attempt a fix, we are happy to point you in the right direction.

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