Short answer. The case starts against that new defendant on the date you filed the later pleading that added them. Rule 1, Section 5 of the Rules of Court provides this, and it applies even if the motion to admit that pleading, where one is needed, is eventually denied by the court.
What the law says
A civil action is commenced by the filing of the original complaint in court.
Rule 1, Section 5 — Commencement of action. Read the full provision →
What the law says
If an additional defendant is impleaded in a later pleading, the action is commenced with regard to him on the date of the filing of such later pleading, irrespective of whether the motion for its admission, if necessary, is denied by the court.
Rule 1, Section 5 — Commencement of action. Read the full provision →
The general rule: commencement follows the original complaint
Rule 1, Section 5 starts with the baseline everyone assumes: a civil action is commenced by the filing of the original complaint in court. For the parties named from the start, that filing date is when the case officially began. But that baseline is not the whole rule, and it is not what applies to a defendant you bring in later through a separate, subsequent pleading.
For an added defendant, the trigger is the later pleading's filing date
The same section addresses your exact situation directly: if an additional defendant is impleaded in a later pleading, the action is commenced with regard to him on the date of the filing of such later pleading. So the case is not treated as having started against that new defendant on the original complaint's date. It starts on the date you filed the pleading that actually brought them in.
This still holds even if the motion to admit is denied
The section adds a specific safeguard for you: the commencement date holds irrespective of whether the motion for its admission, if necessary, is denied by the court. If your later pleading required a motion for leave to file it, and that motion is later denied, the filing date you already secured for commencement purposes is not undone by that denial.
Why the filing date matters even before the pleading is admitted
By fixing commencement to the filing date rather than to the date of admission, the rule protects whatever value that filing date carries for you, without waiting on a ruling that could come later and could go either way. That is the point of writing the rule to apply regardless of the outcome of the admission motion: the date you filed is what counts as the start against that added defendant, whatever the court later decides about the motion itself.
Why this matters practically
This distinction matters most when time is tight, for instance where a limitation period is running against the new defendant. Because Section 5 fixes commencement to the filing date rather than to the date the court rules on admission, you are not penalized for the time a motion for leave takes to be resolved. The rule treats the pleading as having done its work against the new defendant the moment it was filed, not the moment the court got around to acting on it.