Postpone effect

Can you postpone the start of a spell effect with the use of durations?

For example:

Spell Name: The Reaper Comes at Dusk PeCo45
R: Touch, D: Sun, T:Ind
You can cast this spell anytime during the day. At dusk, the victim dies.
(Base 30, +1 Touch, +2 Sun)

You can use vim effects to do this. the "spell container" guideline is appropriate. This is the guideline used for waiting spell and watching ward rituals in the core book. In the core book it tells us that these spells require a ritual because of their potentially indefinite duration. In Rival Magic it further specifies that hermetic magic can't set up a spell that triggers under specified conditions without a ritual. This was probably a good move because setting up non-ritual duration moon or sun versions of waiting spell was exceptionally powerful and made things a bit easy for the characters.

However you could still make a container spell that holds its target spell for its duration then releases it. There is a duration concentration example in Magi of Hermes (Patient Spell IIRC). Here is a version I did a month or so ago on this forum using duration Hours from celestial magic (spell of punctuality fourth post down on this page)

For your example a duration sun spell would work perfectly and I think that a version with the Faerie magic duration of until would be excellent (It would in fact be very similar to the faerie duration of bargain).

You could argue that instead of an associated vim spell one could instead use a special duration. Enchanted items and non-spontaneous magic are not restricted to the ranges durations and targets native to Hermetic magic. They can use unique spell parameters. To date in the published material this has been done by taking the closest equivalent parameter (being harsh in judgement of what is the closest equivalent) and then adding a magnitude so a spell with a "special" duration of fortnight would be five levels higher then one with a duration of moon.

I don't think that you'd win that argument though. A delayed effect is a pretty involved thing and I don't see it as just a weird duration. I'd allow it as a breakthrough of the easiest type. (Is lesser the correct term?)

Something about the effect as written in OP rubs me the wrong way, specifically the Perdo effect not being momentary. Using Sun duration is obviously needed, and I think a complexity mod of +5 levels might be needed. Given Formu magic can also use nonstandard ranges, durations, and targets.
That said, I feel like the delay effect is almost "paid for" in the extended duration to Sun too. Conflicted.
So the complexity bump is the easy way out, which lands on the harsh side. But it also keeps to the sub effect guidelines too.
Having the effect on the target for the day also allows for it to potentially be removed, and I could see a nefarious npc using this as brute force motivation.

(Edit: Thinking about this a bit more, I'd encourage the player to use a combination of a Vim effect with a regular PeCo effect. I'm not sure what can or worms it opens, but something about it makes me nervous.)

Merinita's Minor Illusion Mystery : Spell Timing offers some of it.

Yeah - the Rego Vim "Delay an effect" guideline (as discussed above) is your best bet. It pretty much does exactly what you're asking for - however, the write-up of Watching Ward (which is the implementation of the effect) does specifically say that it is a ritual due to the potentially-infinite duration of the effect. - that is, watching ward lasts potentially forever, until the Trigger for the effect is enacted.

However, that implies that non-infinite durations are possible. As Eric Tyrrell mentions, you could simply have a "Duration: Sun" delay effect - at which point the spell releases at sunrise or sundown. I would argue that you could create a non-ritual Circle/Ring ward, ("Lasts until the circle is broken"). Both the "potentially infinite duration" issue, as well as "breaking a ward", are both well-understood effects already built into Hermetic Theory. However, I would qualify that with "it's not just crossing the boundary of the ward - it's literally breaking it" - so you'd likely have to design the circle so that it was easy to break as your target crosses over it. This may or may not be easy, depending on how your troupe defines "breaking a circle".

Perhaps a more flexible combination is the Concentration duration on the "Delay Effect" - which means you can link it up to a "maintain the demanding spell (ReVi)" - so you can set the actual duration to anything you have a Demanding Spell variant for. (Arc/Day/Month).

Before Rival Magics came out, I usually interpreted that non-Ritual Delay Effects could have triggers - but if the trigger didn't occur, they would fade naturally at the end of their normal duration, and both the ward and the spell inside the ward would simply not go off. I also determined that a ward without an additional Intelligo spell built into it simply had the senses that the magi who cast it did. So you could design a default Watching Ward to go off if someone entered the room - but if the magi was blind, then you couldn't use Sight to define it - you'd have to use Touch or smell or Hearing, or something like that.

EDIT - actually, now that I think about it...a non-trigger watching ward really isn't a watching ward. It's not triggering on anything, so much as the "delay effect" part is merely expiring. After which the spell goes off as normal.

"Ring: The spell lasts until the target of the spell moves outside a ring drawn at the time of casting, or until the ring is physically broken"

You could do a Sun/Ring suppression on a Sun/Ind spell. As soon as you break the Ring, the suppression stops. If you wait for the Sun to go down, there's nothing to suppress anymore. But breaking the Ring at the last second is cheaper than going Moon/Ind.