XSLT 1.0 first date of month

[juls_pro_37]

thx

<xsl:template match="BillingPeriod">
  <xsl:copy>
    <xsl:variable name="BillingPeriod" select="."/>
   <xsl:value-of select="concat(substring(.,1,8),'01')" /> 
  </xsl:copy>
</xsl:template>