time period:time shift parameter is used by functions:
percentile (/host/key,time period<:time shift>,percentage)
and
trendavg (/host/key,time period:time shift)

However, the menu box for entering it differs in many ways:
percentile():

trendsavg():

1) First of all, the first says "Period shift/Period" and another says "Time shift/Time" as the name of the entry box.
But this is the same functional parameter! It has the same name in the documentation signature and putting "now-1h" for example - results in the same expression.

2) Secondly, the greyed out default suggestion for percentile() says:

now-h

but for trendsavg()

now/h

neither now/h or now-h are legal entries, so I am not sure what is the point of them
can those be something like now-1h which would be at least an acceptable entry?

3) Thirdly,
trendsavg() does not allow to put 1 for "Last of (T) Time", and requires units like 1h
but percentile() does allow to put 1 for the "Last of (T) Time" and does not require units