Before you merge, you need to know that all datetime arrays have a date and time associated with them, even if that information is not displayed due to the formatting applied.
d = datetime('today')
d.Format = 'HH:mm:ss'
The point, then, is to first check your format to make sure the information is not already there. If there truly is no date information associated with the time data, then it is best to import this data as a duration instead of a datetime.
As a duration, you could then just add the date and time together.
a = datetime('today')
t = duration(19,20,3)
dt = a+t
However, if your time is a datetime, you must extract just the time of day before adding it to your date.
f = datetime('today')
g = datetime('now','Format','HH:mm:ss')
Y = f+timeofday(g)
