I have tried to simplify this issue as much as I can at the moment: it looks like a bug to me but perhaps I've missed something obvious.
I am trying to build an interactive scheduler using TimelinePlot and I find that the following code reports that I've tried to plot an invalid dataset:
Manipulate[Column@{DatePlus[{2019, 1, 15}, sdate],
TimelinePlot[{{Labeled[{2011, 1, 1}, "hello"], {2012, 1, 1}, {2013, 1, 1}},
{Labeled[Interval[{{2011, 3, 1}, {2012, 7, 7}}], mylabel],
Interval[{{2012, 9, 12}, {2013, 12, 21}}]}, {Labeled[
Interval[{DatePlus[{2011, 3, 1}, sdate],
DatePlus[{2012, 7, 7}, sdate]}], "Moving interval"],
Labeled[Interval[{{2012, 9, 12}, {2013, 12, 21}}], "Range"]}}]},
{sdate, 0, 200, 1},
{mylabel, {"la", "lb", "lc"}}]

but this works fine
Manipulate[Column@{DatePlus[{2019, 1, 15}, sdate],
TimelinePlot[{{{2011, 1, 1}, {2012, 1, 1}, {2013, 1, 1}},
{Labeled[
Interval[{{2011, 3, 1}, {2012, 7, 7}}], mylabel],
Interval[{{2012, 9, 12}, {2013, 12, 21}}]},
{Labeled[
Interval[{DatePlus[{2011, 3, 1}, sdate],
DatePlus[{2012, 7, 7}, sdate]}], "Moving interval"],
Labeled[Interval[{{2012, 9, 12}, {2013, 12, 21}}],
"Range"]}}]}, {sdate, 0, 200, 1},
{mylabel, {"la", "lb", "lc"}}]

as well as this:
With[{sdate = 1, mylabel = "foo"},
Column@{DatePlus[{2019, 1, 15}, sdate],
TimelinePlot[{{Labeled[{2011, 1, 1}, "hello"], {2012, 1, 1}, {2013,
1, 1}}, {Labeled[Interval[{{2011, 3, 1}, {2012, 7, 7}}],
mylabel],
Interval[{{2012, 9, 12}, {2013, 12, 21}}]}, {Labeled[
Interval[{DatePlus[{2011, 3, 1}, sdate],
DatePlus[{2012, 7, 7}, sdate]}], "Moving interval"],
Labeled[Interval[{{2012, 9, 12}, {2013, 12, 21}}], "Range"]}}]}]

