The CMOS battery is dead. On these, the battery is actually user serviceable (rare with Canon; they seem to go back and fourth a lot so when you have this you did well) so it can be done quickly:
Canon PowerShot SX160 IS Clock Battery Replacement
As far as buying the CR1220 for the swap your best bet is batteries+bulbs. This is a weird battery you do not see in most big box stores. You may get lucky at a drug store like CVS or Walgreens but this is not something you will find at Walmart.
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3