[flow] rewrite the broken mDateSplit function

Thing is, it parsed dates incorrectly, e.g. "1 18 8" -> "1188-01-01
00:00:00 UTC"
4 jobs for dev-threaded-flow-with-new-ngrams-extraction in 50 minutes and 31 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Cabal
passed #19531
cabal

00:23:26

 
  Stack
passed #19532
stack

00:02:49

 
  Bench
manual #19533
allowed to fail manual
bench
 
  Test
passed #19534
test

00:24:15