public Builder clearWindowStartTime() {
      windowStartTime_ = getDefaultInstance().getWindowStartTime();
      bitField0_ = (bitField0_ & ~0x00000004);
      onChanged();
      return this;
    }
<pre>
The current start time of the maintenance window.
This timestamp value is in RFC3339 text format.
</pre>

<code>optional string window_start_time = 6;</code>

@return This builder for chaining.
java-tpu/proto-google-cloud-tpu-v2alpha1/src/main/java/com/google/cloud/tpu/v2alpha1/UpcomingMaintenance.java