An attempt to explain the time travel mechanics in Time of Your Life by Emmie. A/N I've debunked my own theory here with this later entry.
"The event happens...(Willow draws a circle in between markers designating the present and the distant future) sometime around here. But it's a temporal anomaly, which means that it affects the timestream, almost exactly like a pebble thrown into a pond...and what you girls are picking up ...is the last subsiding ripple of the event. An event completely fluid temporally..." - Willow ToYL 1
The temporal anomaly starts at one point - a huge event in the future that we are not yet aware of - and creates ripples. The final ripple that goes both into the past and into the future is the one that opens the portal in NYC during present time and the portal in Haddyn in the future. If I'm interpreting this correctly, this temporal anomaly is triggered by an event in the future that opens the portal between the present and the far-off future lines Willow has drawn. This means that an event occurring between Buffy's present and Fray's future is what sets off this temporal anomaly.
If you look at Issue 16 where Willow is explaining about the temporal anomaly, she establishes that the causality occurs from this mysterious event that creates ripples. The outermost ripple is what opens the portal in NYC and transports Buffy to Haddyn.
Willow draws multiple lines that represent ripples. There were actually four different openings of the portal at different times during the chronological timeline of ToYL and they are each connected to their corresponding mirror-portal in the past and future. Portal 1, when Buffy is first taken from NYC, which leads to Portal 3, when Buffy first arrives in Haddyn. Portal 2, when Willow is blindfolded to attempt to rescue Buffy in NYC, which connects to Portal 4, when Future Willow stands in front of the opening portal and Buffy kills her to get through it. The portals are slightly distorted mirrors, not connecting perfectly as Portal 1 leads to Portal 3, but if the ripples were moving outwardly at the exact same rate, Portal 1 would presumably connect to Portal 4. It's a slight deviation, so try to imagine the ripples extending from the temporal event as not perfectly cylindrical, with the mirroring ripples extending slightly further into the past or future upon each opening. The exact time-position of the corresponding mirror portals is less important than the nature of it's motion - moving outward.
Portals 1 and 2 in NYC are separated approximately by one day. Portals 3 and 4 in Haddyn also seem to be separated by one day. Present Willow has *no* control over these portals and consequently, it appears that the Future Willow in Haddyn is also not responsible for opening them. Although it appears she might have taken advantage of the portals to magically send the demon to NYC and magically pull Buffy through the portal. They open on their own and are caused by an event that disrupts time, creating the ripples. At each ripple moving outward from the event, connecting portals between the past and the future are opened.
If portals 1 and 2 are mirrors of portals 3 and 4, then the temporally displacing event must occur after Buffy returns to NYC (portal 2) but before she arrives in Haddyn in the future (portal 3).
This means that Future Willow's death (as some fans have been theorizing) is not the cause of the temporal anomaly because it occurs after portals 3 and 4 are opened.
So the next question is - what event caused this temporal anomaly and how far into the future does it occur?
Imposing causality on a temporal fold is severely problematic, but the one clear rule given to us by Willow is that the one event is the origin for these outwardly moving portals. The temporal anomaly occurs at some point in the future and causes change in the past and the future. The effect of this event acts outside of the chronological timeline and consequently isn't subject to it's linear progression. It *upsets* linear progression. It's the timeline rule-breaker. In the game of Rock, Paper, Scissors - the temporal anomaly is paper and the chronological timeline and it's subsequent rules of causality are rock. Paper always covers rock.
The beauty of these portals *connecting* resonates thematically as connection is one of the main themes of Season 8. Buffy's connection to Willow, Buffy's connection to her slayers, Buffy's connection to Satsu, the chain that so strongly connects all slayers and conversely leads to Buffy's disconnection from the world outside her Slayer army. The portals physically connect Buffy to Fray, connecting Buffy's present to Fray's future and paralleling the greater goal of Season 8 - connecting the events of Chosen to Fray. Connection - it's the whole point.