Consider: "date -d '2017-03-28 17:12:34 + 3 hours'"

In "date (coreutils) 5.2.1  (March 2004)" it gives "Tue Mar 28 20:12:34 
PDT 2017".

In "GNU coreutils 8.22 February 2016)" it gives "Tue Mar 28 08:12:34 PDT 
2017".

Both servers use /etc/localtime -> /usr/share/zoneinfo/PST8PDT

If I insert "PDT" in the original string after the time, it works on 
both servers.  Unfortunately, since the servers run in an area that is 
subject to daylight savings time part of the year, hard coding the 'PDT' 
zone in the script it was in, is not appropriate.

On both servers, "date -d '2017-03-28 17:12:34 3 hours ago'" works. Is 
there a "3 hours ahead"-type phrase???

I have settled on "... 21 hours ago tomorrow", which works on both servers.