The OSTC doesn't store the start of the dive, but the exit time. Hence
the dive time needs to be substracted.
For dives with format version 0x21, we prefer the total dive time in
seconds stored in the extended header. This time value also includes the
shallow parts of the dive, and therefore yields the most accurate start
time. The dive time is rounded down towards the nearest minute, to match
the value displayed by the ostc. For dives with the older format version
0x20, this value isn't available and we default to the normal dive time.
