+1 to using java.util.calendar. Plus the java.util.date is becoming more and more depreciated, as a rule it really shouldn’t be used.
# Calendar Class has built in addition/subtraction of dates, also the Date class is largely depreceated.
from java.util import Calendar
# Create a calendar with the current time
rightNow = Calendar.getInstance()
#To add of subtract hours
# To add or subtract weeks
# To add or subtract months
# To add or subtract years
#To assign a date/time property in Ignition (such as a trend start date) with the new time
trend = event.source.parent.getComponent('TREND')
trend.startDate = rightNow.time