- Use time_t instead of abstract minutes to store validity time to make sure we support all possible values.
- Return UNIX time from it to support both relative and absolute expire times.
- Pass a current time to gsm340_validity_period() to compute the absolute expire time if a relative time is specified. What time is "current" is to b
- We also introduce SMS_DEFAULT_VALIDITY_PERIOD macros to reduce number of magic values.
PS This is a first patch of a series of patches which fix SMS validity time decoding. I'm cleaning the patch set and want to see if this patch is fine, as other patches rely on it.
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