Given a timestamp of
2019-12-12, you can specify
++15d (note the double plus) to move forward to
2019-12-27, then you can specify
++1m to move forward to
2020-01-27. See the section of the manual entitled The date/time prompt:
With a single plus or minus, the date is always relative to today. With a double plus or minus, it is relative to the default date.
EDIT: Here's a more detailed workflow. The simplest method I know to do what you want is to use the date/time prompt and select your initial date from the calendar: e.g. for a SCHEDULE: date, say
C-c C-s and click on 2019-12-12. That makes 2019-12-12 the default date, so you can say
C-c C-s again and type
++15d at the date/time prompt: that gets you to 2019-12-27 which is now the new default date. So you can do
C-c C-s one more time and say
++1m at the prompt to get you to 2020-01-27. Does that help?
Note that with
C-c ., every time you modify the date, the cursor ends up after the date, so if you do it again, you end up with a date range. Just move the cursor back into the date before repeating it and you will not get a second date.