Fix BSON load error by saving timestamp as string

[dahong67]

To resolve errors where BSON doesn't manage to reconstruct the DateTime.

[codecov]

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✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (4d8de84) to head (6a699bb).
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@@            Coverage Diff            @@
##            master       #31   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files            3         3           
  Lines           57        57           
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  Hits            57        57           

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[dahong67]

In particular, observed the following behavior on CacheVariables v0.2.0:

(jl_gaMG15) pkg> st
Status `/private/var/folders/h_/3320nshd7t1bzbtcrj023bkr0000gn/T/jl_gaMG15/Project.toml`
  [9a355d7c] CacheVariables v0.2.0

julia> using CacheVariables

julia> cache("test.bson") do
           a = "a very time-consuming quantity to compute"
           b = "a very long simulation to run"
           return (; a = a, b = b)
       end
┌ Info: Saved cached values to test.bson.
│   Run Timestamp : 2026-01-19T14:59:26.331 UTC (run took 0.003582958 sec)
└   Julia Version : 1.12.4
(a = "a very time-consuming quantity to compute", b = "a very long simulation to run")

julia> cache("test.bson") do
           a = "a very time-consuming quantity to compute"
           b = "a very long simulation to run"
           return (; a = a, b = b)
       end
ERROR: UndefVarError: `Dates` not defined in `Main`
Suggestion: check for spelling errors or missing imports.
Hint: Dates is loaded but not imported in the active module Main.
Stacktrace:
[...]

Related to: #28

I think the issue here may be that Dates has been loaded in CacheVariables but that's (probably?) not the right module if we want things like #28 (i.e., DataFrames in Pluto) to work. Not sure... Tried a few fixes but didn't get something to work.

Anyway, the simple fix is probably to just store the date as a string in BSON.

[dahong67]

Notably, JLD2 seems to work fine! So, no need to switch to strings for that one.

julia> cache("test.jld2") do
           a = "a very time-consuming quantity to compute"
           b = "a very long simulation to run"
           return (; a = a, b = b)
       end
┌ Info: Saved cached values to test.jld2.
│   Run Timestamp : 2026-01-19T15:04:01.087 UTC (run took 0.007473458 sec)
└   Julia Version : 1.12.4
(a = "a very time-consuming quantity to compute", b = "a very long simulation to run")

julia> cache("test.jld2") do
           a = "a very time-consuming quantity to compute"
           b = "a very long simulation to run"
           return (; a = a, b = b)
       end
┌ Info: Loaded cached values from test.jld2.
│   Run Timestamp : 2026-01-19T15:04:01.087 UTC (run took 0.007473458 sec)
└   Julia Version : 1.12.4
(a = "a very time-consuming quantity to compute", b = "a very long simulation to run")

Once I've gotten to play around with JLD2 to get some more confidence in it, I think we should probably switch all the examples to use that (i.e., encourage that as the default rather than BSON).