PS Code for Dynamically setting a build number.

TeamCityBuildNumber.ps1

# The build number format is YYYY.MM.B (Year.Month.BuildCounter)

$MonthCounter = ([DateTime]::Now.Month).ToString().PadLeft(2, "0")
$YearCounter = ([DateTime]::Now.Date.Year).ToString()
$DayCounter = ([DateTime]::Now.Day).ToString().PadLeft(2, "0")
$TimeCounter = ([DateTime]::Now.Hour).ToString().PadLeft(2, "0")
$MinuteCounter = ([DateTIme]::Now.Minute).ToString().PadLeft(2, "0")

$counter = "$YearCounter.$MonthCounter.$DayCounter.$TimeCounter" + $MinuteCounter

<# writing the build number back to the host to overwrite the variable for Team City #>
Write-Host "##teamcity[buildNumber '$counter']"

heads-up! you could do this with one line

<# writing the build number back to the host to overwrite the variable for Team City #>
Write-Host ("##teamcity[buildNumber '{0}']" -f [datetime]::Now.ToString("yyyy.MM.dd.HHmm"))