Valmiki Ramayana Ayodhya Kanda Sarga 62 Shloka 18

वनवासाय for exile in the forest, रामस्य Rama's, पञ्चरात्रः five nights, अद्य today, गण्यते has been counted, यः such a period, शोकहतहर्षायाः whose joy has been destroyed by sorrow, मम to me, पञ्चवर्षोपमः equal to five years.

This is the fifth night of Rama's exile, but it is like five years to me whose joy has been destroyed by sorrow.