Source: German Church Records of Westmoreland County vol 3, by Paul Miller Ruff, available from http://www.abesbooks.com
My family's "brick wall" was found with Samuel HAGAN, born Feb. 14, 1811. Baptism records in the book cited above show a Samuel HAGAN, b Feb. 20, 1810, baptised Sep. 21, 1812. Two appearent siblings were also baptised that day, Sara, b Feb. 13, 1803 and Johannes (John?), b. Aug 5, 1807. Parents are listed as Etwart (Edward) and Elissabetha (Elizabeth) HAGAN. Edward HAGAN appears in the census records for Armstrong CO in 1810 and 1820.
Other Hagans are listed, including David and Elizabeth, with the last name the variant "HEGEN".
I found the German Reformed Church may have celebrated the start of the new year after the Annunciation, May 25, which could account for the 1810 / 1811 discrepancy in Samuel's record. The 6 days separating the day may also be due to different adoptions of the change to the Gregorian calendar which Britain adopted in the mid-1750s. I am waiting for more info on the calendar in use by the German Reformed Church at that time.