Hi everyone, I need to do calculations on millions of timestamps, such as

t0 = "2011-01-11 11:30:01.321"

If I want to subtract 12 hours from the timestamp and convert the result to a list of date items, I could use DatePlus and DateList like this

DateList[DatePlus[t0, {-12, "Hour"}]]

and it takes {0.003359,{2011,1,10,23,30,1.}} on my MacBook Pro. But it is apparently a lot faster subtracting the 12 hours this way

DateList[DateList[t0] + {0, 0, 0, -12, 0, 0}]

taking only {0.000801,{2011,1,10,23,30,1.321}}, where the outer DateList conveniently adjusts the day so that the hours position shows a positive integer. The second way also keeps the fractions of a second (1.321) where the first way drops them.

I'm leaning towards doing it the second way, and am curious to know whether anyone see a downside.