Comment by ytx

I don't have monthly data, but as an approximation here's a rough test where you make a one time investment of $1000, either all at once, or equally spaced over 2 or 5 years. This is simulated starting at each year from 1985 to 2005, and we count the number of years underwater starting 5 years after the first year (after $1000 has been put in for all 3 "strategies") up until 2023.

  S&P:
         once  dca_2yr  dca_5yr
  count  25.0     25.0     25.0
  mean    0.7      0.8      0.7
  std     1.8      1.5      1.1
  min     0.0      0.0      0.0
  max     8.0      6.0      3.0

  Nikkei:
         once  dca_2yr  dca_5yr
  count  25.0     25.0     25.0
  mean   11.4     11.5     11.8
  std     9.1      9.5      9.4
  min     0.0      0.0      0.0
  max    29.0     29.0     28.0

So investing at once, the max number of years underwater for the S&P was 8, versus 3 when "dca"ing over 5 years. The average number of years underwater (averaged over when you would've invested) is quite low, while for the Nikkei all metrics look much worse.