You're already using expressions. Date.Now is an expression that gives you the current time in unix timestamp format. Date.GetUTCHours gets you the hours from a timestamp, so you put Date.Now in Date.GetUTCHours, resulting in Date.GetUTCHours(Date.Now)
Now if you want to zeropad the resulting number, you use the zeropad expression. It would be zeropad(Date.GetUTCHours(Date.Now),2), where Date.GetUTCHours(Date.Now) is the number you want padded and 2 is the amount of padding.