cron-parser is a Lua wrapper of ccronexpr C library. It parses a cron-format schedule and calculates the nearest event date.
tarantoolctl rocks install cron-parser
local cron = require('cron')
local expr, err = cron.parse('0 */15 * * * *')
if not expr then
print('Error: '..err)
end
local ts = cron.next(expr)
print(os.date('%Y-%m-%d %H:%M:%S', ts))
ccronexpr library: https://github.com/staticlibs/ccronexpr