Constant BOOT_EXPR_PS1 

pub const BOOT_EXPR_PS1: &str = "[math]::Floor([DateTimeOffset]::new((Get-CimInstance Win32_OperatingSystem).LastBootUpTime).ToUnixTimeSeconds() / 60) * 60";

The spec’s boot value: LastBootUpTime as epoch seconds truncated to the minute (the same expression SystemProbe::boot_id runs).