This powershell is being used as a Teamcity Build Step that sets the BuildNumber as Date.Counter (i.e 2018.02.02.1) and resets counter everyday. In collaboration with @axelwass

SetBuildNumber.ps1

#$teamcityUser = '%teamcity.api.username%'
#$teamcityPass = '%teamcity.api.password%'
#$teamcityUrl = '%teamcity.api.url%'

[System.Net.ServicePointManager]::SecurityProtocol = [System.Net.SecurityProtocolType]::Tls12
$teamcityUser = '****'
$teamcityPass = '****'
$teamcityUrl = 'yadayada.com'
$buildId = '123456'
$projectId = 'ProjectId'

function GetBuildCount
{
    param
    (
        [string] $ServerUrl,
        [string] $UserName,
        [string] $Password,
        [string] $BuildId,
        [string] $ProjectId
    )
 
        $client = New-Object System.Net.WebClient		
        $pair = "$($UserName):$Password"
        $encodedCreds = [System.Convert]::ToBase64String([System.Text.Encoding]::ASCII.GetBytes($pair))
        $client.Headers.Add("Authorization", "Basic $encodedCreds")
        $date = Get-Date -Format yyyyMMddT000000zz00
        $dateEncoded = [uri]::EscapeDataString($date)        
        $url = "https://$ServerUrl/httpAuth/app/rest/buildTypes/id:$projectId/builds?locator=sinceDate:$dateEncoded,defaultFilter:false"        
        $response = [xml]$client.DownloadString($url)
        $builds = $response.builds.build | ? {$_.id -le "$BuildId"}        
                
        $count = $builds.count        

        if (!$count) {
            $count = 1
        }

        return $count
}

$buildCount = GetBuildCount -ServerUrl $teamcityUrl -UserName $teamcityUser -Password $teamcityPass -BuildId $buildId -ProjectId $projectId

Write-Host "##teamcity[buildNumber '$([DateTime]::UtcNow.ToString("yyyy.MM.dd")).$buildCount']"

Lines 33 to 37 are a workaround for empty value on the first build of the day

Huge thanks to you guys!