Starting date of FMLA period
The federal regulation applicable to the FMLA (29 CFR 825.208) provides:
(b)(1) Once the employer has acquired knowledge that the leave is being taken for an FMLA required reason, the employer must promptly (within two business days absent extenuating circumstances) notify the employee that the paid leave is designated and will be counted as FMLA leave.
(c)If the employer has the requisite knowledge to make a determination that the paid leave is for an FMLA reason at the time the employee either gives notice of the need for leave or commences leave and fails to designate the leave as FMLA leave (and so notify the employee in accordance with paragraph (b)), the employer may not designate leave as FMLA leave retroactively, and may designate only prospectively as of the date of notification to the employee of the designation. In such circumstances, the employee is subject to the full protections of the Act, but none of the absence preceding the notice to the employee of the designation may be counted against the employee's 12-week FMLA leave entitlement.
Therefore, your 12 week period began as of the date the employer notified you of the FMLA leave.