documentation - Correctly using GTFS realtime data arrival time -

this question has nothing parsing data gtfs realtime data (been there, done that). have started experimenting realtime data nyc's mta, , have become little confused arrival time's meaning. according gtfs-realtime reference new york city subway,

message stoptimeevent

fields

field name | nyc subway usage

delay       | not used time        | defined in gtfs-realtime specification uncertainty | not used 

another useful note found under tripupdate:

[stop_time_update] includes future stop times trip stoptimes past omitted. first stoptime in sequence stop train approaching, stopped @ or leave. a stop dropped sequence when train departs station.

^{emphasis mine}

after short searching, found "gtfs-realtime specification" mentioned in first quote. on page stoptimeevent, says,

use time whether there predicted schedule or not. if specify both time , delay, time takes precedence. normally, time, (if given scheduled trip) should equal gtfs-specified departure time (scheduled_time) plus delay.

under that, there table similar 1 above, time has more useful detail:

time | int64 | optional | event absolute time, in posix time (seconds since january 1st 1970 00:00:00 utc). 

however, when mintoarrival = (posixtime [from update] - currentposixtime) / 60, many positive and many negative values. not make sense since, according mta, stop times past omitted, shouldn't have negative value -89 min (which do). have large positive values 224 min. these huge values –– , positive and negative –– makes me think misinterpreting quotes.

how 1 make sense of (in opinion) scant documentation , intended/actual minutes until arrival?