Add --delta option

[hollasch]

To enter a single-valued time delta, in ISO 8601 format. Note that this precludes any other time values on the command line.

[hollasch]

Needs to handle nominal vs. tropical years.

[hollasch]

Maybe something like 23t114d13h17m88s or 23t 114d 13:17:08.

[hollasch]

The --delta option should follow the ISO-8601 duration format. The leading P should be optional (implied through use of the --delta option). Based on my understanding, the date components suggest simply adding/subtracting the delta field-by-field to the current date-time.

In addition, the --delta option should allow an optional +/- sign either immediately before or immediately after the leading P character.

[hollasch]

A fixed time plus a delta time yields a fixed time. For example, this would allow you to print the time one tropical year from some given date. In addition, a single delta time yields a delta time. This all suggests that the one-versus-two time specifications logic would need to change in several cases.

fixed → fixed
delta → delta
delta+ → delta
fixed, fixed → delta
fixed, fixed, delta+→ delta

[hollasch]

Note: if --delta is provided in conjunction with an absolute time (--now, --access, --creation, --modification, --time), then it is applied to the given time to yield an absoute time. This provides a way to find the time 70 days from now, for example, or 43 seconds before a file's modification time.