LastPwdSEt is Not Updating by Updating a password - sharepoint

Question

I like to calculate the passowrd expiration date for the user in active directory
I am able to get the MaxPWdAge and LastPwdSet attributes.
But the problem is whenever i am changing the password of one of a user for testing purposes, lastPwdSet is not updating and it is still showing the old date?
Can anybody tell me why is this happen ?
public bool CheckPassWordExpiryDate(string LdapPath, string Username, string Password)
{
DomainConfiguration domainConfig = new DomainConfiguration();
// Configuration(web.config) changes
DirectoryEntry de = new DirectoryEntry("LDAP://" + LdapPath, domainConfig.UserName, domainConfig.Password);
DirectoryEntry entry = new DirectoryEntry();
entry.Username = Username;
entry.Password = Password;
//Function to get maximum password age from the active directory
int maxPwdAge = GetMaxPasswordAge();
// Function to get last password set date for the use.
DateTime pwdLastSet = GetPwdLastSet("pwdLastSet", Username);
//Add maximum password age days to Last password set days , if it is less than today's date means that password has been expired else it is not expired
if (pwdLastSet.AddDays(maxPwdAge) < DateTime.Now)
{
return true;
}
else
{
return false;
}
}
public static int GetMaxPasswordAge()
{
DomainConfiguration domainConfig = new DomainConfiguration();
using (new SPMonitoredScope("AD Properties"))
{
using (DirectoryEntry domain = new DirectoryEntry("LDAP://" + domainConfig.DomainName, domainConfig.UserName, domainConfig.Password))
{
DirectorySearcher ds = new DirectorySearcher(
domain,
"(objectClass=*)",
null,
SearchScope.Base
);
SearchResult sr = ds.FindOne();
TimeSpan maxPwdAge = TimeSpan.MinValue;
if (sr.Properties.Contains("maxPwdAge"))
maxPwdAge = TimeSpan.FromTicks((long)sr.Properties["maxPwdAge"][0]);
return maxPwdAge.Duration().Days;
}
}
}
public DateTime GetPwdLastSet(string attr, string UserName)
{
DomainConfiguration domainConfig = new DomainConfiguration();
using (new SPMonitoredScope("AD Properties"))
{
using (DirectoryEntry domain = new DirectoryEntry("LDAP://" + domainConfig.DomainName, domainConfig.UserName, domainConfig.Password))
{
//DirectorySearcher searcher = new DirectorySearcher(domain, "(|(objectClass=organizationalUnit)(objectClass=container)(objectClass=builtinDomain)(objectClass=domainDNS))");
DirectorySearcher searcher = new DirectorySearcher(domain);
searcher.PageSize = 1000;
searcher.Filter = "(SAMAccountName='" + UserName + "')";
searcher.Filter = "(|(objectCategory=group)(objectCategory=person))";
var user = searcher.FindOne();
DateTime pwdLastSet = DateTime.FromFileTime((Int64)user.Properties["PwdLastSet"][0]);
return pwdLastSet;
}
}
}
} }