Rather than using datetime, the Scripting / Python / Python in Ignition / Working with Different Datatypes section of the manual points out it is easier to use the Java java.util.Date class.
The following script should let you do what you want:
# Use the java Date class, the same as a memory tag of type DateTime.
from java.util import Date
# This returns the current time.
now = Date()
# Read the value of the memory tag.
lastDatetime = system.tag.read('New Tag').value
# The getTime() function returns the DateTime in milliseconds, so you
# can subtract one from the other. The result is in milliseconds.
tdMillis = now.getTime() - lastDatetime.getTime()
# If you wish you can create another Date object from the millisecond value.
td = Date(tdMillis)
# The dateFormat command allows you to format the Date object any way you want.
print tdMillis, system.db.dateFormat(td, "mm:ss.S")