Like the US and unlike many other countries, Singapore visas' expiration date is the last date which you may enter the country. It is not the date which you must leave. The allowed duration of your stay will be stamped into your passport by immigration officials.
From the Immigration & Checkpoints Authority:
A Singapore visa is not an immigration pass. It is a pre-entry permission for the holder of a valid Singapore visa to travel to, and seek entry, into Singapore. The grant of an immigration pass will be determined by the Immigration & Checkpoints Authority (ICA) officers at the point of entry. Possession of a valid visa alone does not guarantee entry into Singapore.
A holder of a valid Singapore visa who is found suitable for entry into Singapore will be issued with an immigration pass to enter and remain in Singapore. You are advised to check your passport for the arrival visit pass endorsement when in Singapore and take note of the period of stay granted before leaving the checkpoint. The period of stay granted is shown on the visit pass endorsement given on your passport and it is not tied to the validity of your visa.