Should be, make sure you mark that time when you want it to start - I’m going into my own theories here and feel free to discount them, but it seems to me that spirits see things in terms of chains of cause and effect, rather than clock time, calendar time, and the mental ideas we have of time as reaching from left to right, or back behind us and forwards ahead of us etc.
Because of that, I’d suggest performing a small sacrifice (a candle or maybe wine, spirits, or something) to mark the date when you want the effects to commence - so the evocation would be something like, “When I sacrifice this (item placed on altar, but sealed/unlit) to you, please begin to manifest these effects, and I will give you a further offering (of whatever you’ve agreed upon) when the results are clearly seen by me.”
So the sacrifice marks the desired beginning in linear time of a chain of events, it’s a candle that can never be un-burned, a drink that can never be used again, something like that.
The reason I recommend having it present at the start is so the spirit can connect to it, it’s not an offering (as in, giving a gift of something) - it’s more like a starter’s pistol.
I’ve used this method because of my theory about how spirits see time, and it’s worked.
Other people have described getting results by using clock and calendar time, and I’m certain they did, but I was never able to until I explored the issue of how spirits may see things differently to us, and started anchoring things not by date but by a cause > effect chain.
So if someone says “Eva’s wrong, specify you need this on the Nth day” they’re probably able to get results by doing that.
(Sorry this is a bit rambly…)