Example repro:

1. emacs -Q
2. C-x * *
3. t N
4. ' 1.5hr RET
5. +

Expected:

1:  <2018-03-22 Thu 02:30>

Got:

1:  <2018-03-22 Thu 01:00> + 1.5 hr


Emacs  : GNU Emacs 27.0.50 (build 7, x86_64-pc-linux-gnu, GTK+ Version
3.22.28)
 of 2018-03-19
Package: Calc