XML : navigating back after filter in xslt select

For the following xml structure, I have a select expression which outputs , my expectation is however the date. Here is xml:

   <PRODUCT_PRICE_DETAILS>                  <VALID_START_DATE>2015-04-01</VALID_START_DATE>                  <VALID_END_DATE>2016-03-31</VALID_END_DATE>                  <PRODUCT_PRICE price_type="net_list">                      <PRICE_AMOUNT>21.6</PRICE_AMOUNT>                      <PRICE_CURRENCY>EUR</PRICE_CURRENCY>                      <TAX>0.19</TAX>                      <PRICE_FACTOR>1.0</PRICE_FACTOR>                      <LOWER_BOUND>1.0</LOWER_BOUND>                      <TERRITORY>DE</TERRITORY>                  </PRODUCT_PRICE>              </PRODUCT_PRICE_DETAILS>              <PRODUCT_PRICE_DETAILS>                  <VALID_START_DATE>2015-04-01</VALID_START_DATE>                  <VALID_END_DATE>2016-03-31</VALID_END_DATE>                  <PRODUCT_PRICE price_type="nrp">                      <PRICE_AMOUNT>21.6</PRICE_AMOUNT>                      <PRICE_CURRENCY>EUR</PRICE_CURRENCY>                      <TAX>0.19</TAX>                      <PRICE_FACTOR>1.0</PRICE_FACTOR>                      <LOWER_BOUND>1.0</LOWER_BOUND>                      <TERRITORY>DE</TERRITORY>                  </PRODUCT_PRICE>              </PRODUCT_PRICE_DETAILS>    

and here the xslt command:

  <Start><xsl:value-of select="PRODUCT_PRICE_DETAILS/PRODUCT_PRICE[price_type='nrp']/../VALID_START_DATE"/></Start>