make COLDSTARTDATE for Gustav replay match an NHC cycle time

[jasonfleming]

Looking at the results for the nowified Gustav (al072008), the COLDSTARTDATE value came out at hour 20Z rather than 18Z or 00Z the following day.

#-- gustav configurations generated by StormReplay.com WebUI --#
BACKGROUNDMET=off
TROPICALCYCLONE=on
  STORM=07
  YEAR=2023
  STARTADV=1
  ENDADV=35
TRIGGER=rssembedded # required mode
  FTPSITE=stormreplay.com
  HDIR=/atcf/btk/82AaeD2EOWvgjZ6fKYaXXHuSxwyh1Q04
  RSSSITE=stormreplay.com:443/rss/82AaeD2EOWvgjZ6fKYaXXHuSxwyh1Q04
COLDSTARTDATE=2023040920
HINDCASTLENGTH=30.0
WAVES=off
CYCLETIMELIMIT="99:00:00"
#-- gustav configurations generated by StormReplay.com WebUI --#

[jasonfleming]

Looking at the nowified BEST track file, it appears that all times are shifted two hours early compared with official cycle times at 00, 06, 12, 18 ... it also looks like the times in the forecast file (index-at.xml) are shifted also. I think this is something we can fix easily ... let's discuss.