public Builder setWindowStartTime(java.lang.String value) {
      if (value == null) {
        throw new NullPointerException();
      }
      windowStartTime_ = value;
      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>

@param value The windowStartTime to set.
@return This builder for chaining.
java-tpu/proto-google-cloud-tpu-v2alpha1/src/main/java/com/google/cloud/tpu/v2alpha1/UpcomingMaintenance.java