[dmw_ng]

Another cheap trick if the time column is sequential is to split the string into date and time components, cache the date part and calculate the time part just with some multiplication

Major caveat is timezone handling, but this only applies in a subset of situations

[quietbritishjim]

If you've got to that point of modifying the storage format then you might as well just use an integer (microseconds success the epoch) and be done with it. That seems cleaner than using a string (or two strings) anyway.