I think this a serious limitation for a great function but why not do something like this:
--where zero is the current value of the unit
CREATE FUNCTION [dbo].[DateTrunc]
(
    @Unit varchar(10),
    @DateTime datetime,
    @Num int
)
RETURNS DateTime
AS
BEGIN

    RETURN
        case left(@Unit, 1)
        when 'y' then dateadd(yy, datediff(yy, 0, @DateTime) -@num, 0)
        when 'q' then dateadd(qq, datediff(qq, 0, @DateTime) -@num, 0)
        when 'w' then dateadd(wk, datediff(wk, 0, @DateTime) -@num, 0)
        when 'd' then dateadd(dd, datediff(dd, 0, @DateTime) -@num, 0)
        when 'h' then dateadd(hh, datediff(hh, 0, @DateTime) -@num, 0)
        when 'n' then dateadd(mi, datediff(mi, 0, @DateTime) -@num, 0)
        when 's' then dateadd(ss, datediff(ss, dateadd(dd, datediff(dd, 0, @DateTime), 0), @DateTime) -@num, dateadd(dd, datediff(dd, 0, @DateTime), 0))
        when 'm' then
        Case when @Unit in ('month', 'mm', 'm')    then dateadd(mm, datediff(mm, 0, @DateTime) -@num, 0)
             when @Unit in ('minute', 'mi')        then dateadd(mi, datediff(mi, 0, @DateTime) -@num, 0)
             else null
        end
        else null
        end;

END

GO