I use POSIXct for datetimes. Is thee a timeAlign function that I can
use where :
align by year
direction -1 ==> start of this year
direction 1 ==> start of next year
align by week
direction -1 ==> date on last sunday
direction 1 ==> date on next sunday
align by day
direction -1 ==> time at past midnight
direction 1 ==> time at this comming midnight