for consistency with the handling of restarts in Time. See
commit 7aca9bfa78
Author: Henry Weller <http://cfd.direct>
Date: Fri Dec 11 13:37:16 2020 +0000
Time: Added beginTime used to rationalise the write times when restarting
Now if a case is restarted from an arbitrary time, for example one generated at
a premature stop condition, or with an increased writeInterval, the subsequent
time directories written are referenced to the original start time of the case
rather than the restart time.
562df25d13