Recently, I came across a weird issue. Our SharePoint site was running using the Locale, US [English]. We’re supposed to change it to UK i.e., to change the site’s DateTime format from mm/dd/yy to dd/mm/yy. One can easily do that in the Regional Settings option of their site. However, after the change, we noticed a discrepancy in the DateTime displayed in all our custom XSLTs.Say, for a Date, Friday, January 02, 2015, the value displayed after the change was Sunday, February 01, 2015! Clearly, it was the issue with the date format. Once the value 01/02/2015 [mm/dd/yyyy] got changed to 02/01/2015 [dd/mm/yyyy], something went wrong in our custom view WebPart. However, a Date value, where the date was more than 12, say, 20/12/2014 [dd/mm/yyyy] was being correctly displayed as, Friday, December 19, 2014 which, again, was very surprising. The current settings of the custom WebPart was:
<span class="submittedby"> <xsl:text>Author:</xsl:text> <xsl:value-of select="@AuthorBy" disable-output-escaping="yes"/> <xsl:text> on </xsl:text> <xsl:value-of select="ddwrt:FormatDate(string(@SomeDateField), 2057, 3 )"/> </span>
where,   (with semi-colon), denotes a single space character.
I was surprised. The LCID 2057[UK] was already being passed to the function, FormatDate, even when the site’s locale was set to US. Then why the webPart would behave correctly for the locale US and not for UK? It took me a while to figure out that the LCID  that was being used. only affects the output from the method, FormatDate. It however, expects the DateTime in a given format only as input, which is, mm/dd/yyyy. So when the site locale was US, everything was working fine as the date returned by SharePoint was already in this format but, when I changed it to UK things got different.
One important thing to not here is, that the FormatDate function tolerates the obvious error like 20/12/2014 [dd/mm/yyyy] which automatically became, Friday, December 19, 2014, which indeed, is quite frustrating. Anyway, since we knew that in our case, the site locale would always remain UK, we applied a hard-code fix to this problem. Since, the method expects DateTime of type string, so instead of directly passing the Date field to it, we decided to construct the DateTime from dd/mm/yy[UK returned by SharePoint] to mm/dd/yyyy[US which the method accepts as the only input format] using the string-split approach. Following is the modified code snippet.
<span class="submittedby"> <xsl:text>Author:</xsl:text><xsl:value-of select="@AuthorBy" disable-output-escaping="yes"/> <xsl:text> on </xsl:text> <!--original date in dd/mm/yyyy without time--> <xsl:variable name="dateFirst" select="substring-before(@SomeDateField,' ')"></xsl:variable> <!--date--> <xsl:variable name="date" select="substring-before($dateFirst,'/')"></xsl:variable> <!--month & Year part--> <xsl:variable name="dateRemaining" select="substring-after($dateFirst,'/')"></xsl:variable> <!--month--> <xsl:variable name="month" select="substring-before($dateRemaining,'/')"></xsl:variable> <!--year--> <xsl:variable name="year" select="substring-after($dateRemaining,'/')"></xsl:variable> <!--construct date as mm/dd/yyyy--> <xsl:variable name="pubDate" select="concat($month, '/', $date, '/', $year)"></xsl:variable> <!--use the date from the variable @pubDate--> <xsl:value-of select="ddwrt:FormatDate($pubDate, 2057, 3 )"/> </span>
After this fix, everything apparently, returned back to normal, however we know that if tomorrow we try to use the same XSLT to a different site whose locale is not set to UK then, the same issue is bound to re-generate and we have to rework on our split logic.