The spec says:
<MTrk event> = <delta-time> <event>
<delta-time> is stored as a variable-length quantity. It represents the amount of time before
the following event. If the first event in a track occurs at the very beginning of a track, or if
two events occur simultaneously, a delta-time of zero is used. Delta-times are always present.
(Not storing delta-times of 0 requires at least two bytes for any other value, and most delta-
times aren't zero.) Delta-time is in ticks as specified in the header chunk.
<event> = <MIDI event> | <sysex event> | <meta-event>
<MIDI event> is any MIDI channel message. Running status is used: status bytes of MIDI
channel messages may be omitted if the preceding event is a MIDI channel message with the
same status. The first event in each MTrk chunk must specify status. Delta-time is not
considered an event itself: it is an integral part of the syntax for an MTrk event. Notice that
running status occurs across delta-times.
It doesn't matter if the file has a single track (like in format 0) or has several tracks, the delta time is always the amount of time before
the following event in that track.